Add fresher-qa module: 50 Java interview questions for freshers, one runnable demo each, asserted transcripts, javac error captures
Co-Authored-By: Claude Sonnet 5.5 <[email protected]> Claude-Session: https://claude.ai/code/session_017ic4b5DfjBGLBr7F5km2AN
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Claude
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=== Q01 Why is Java platform independent? ===
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magic = CAFEBABE
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class file major version = 69
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=== Q02 JDK vs JRE vs JVM ===
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running on Java 25
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JVM: OpenJDK 64-Bit Server VM
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compiler available: true
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=== Q03 Primitive types and their sizes ===
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byte 1 byte, -128 to 127
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short 2 bytes, -32768 to 32767
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int 4 bytes, -2147483648 to 2147483647
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long 8 bytes, -9223372036854775808 to 9223372036854775807
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float 4 bytes, double 8 bytes, char 2 bytes
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field defaults: 0, false, null
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=== Q04 Type casting and overflow ===
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7 / 2 = 3, (double) 7 / 2 = 3.5
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(byte) 130 = -126
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Integer.MAX_VALUE + 1 = -2147483648
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c + 1 = 66, (char) (c + 1) = B
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0.1 + 0.2 = 0.30000000000000004
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Math.addExact: integer overflow
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=== Q05 Autoboxing and the Integer cache ===
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127 == 127 -> true
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128 == 128 -> false
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128 equals 128 -> true
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Long 127 equals Integer 127 -> false
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=== Q06 Is Java pass by value or pass by reference? ===
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after reassign: original
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after mutate: original + mutated
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=== Q07 static variables, static blocks and initialisation order ===
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static block runs once, when the class loads
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instance block runs before each constructor body
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constructor #1
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instance block runs before each constructor body
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constructor #2
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=== Q08 final, finally and finalize ===
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final array contents changed: 99
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in try
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finally always runs
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Object.finalize forRemoval = true, since 9
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=== Q09 Stack vs heap ===
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heap array length = 1000000
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StackOverflowError after many frames: true
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=== Q10 The four pillars of OOP ===
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Circle area = 3.14
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Rectangle area = 6.00
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rejected: amount must be positive, balance still 500.0
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=== Q11 Inheritance and constructor chaining ===
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Animal constructor
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Dog constructor
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Animal constructor
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Puppy constructor for Bruno
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=== Q12 Method overloading vs overriding ===
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FancyPrinter.show(Object)
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show(String)
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FancyPrinter.show(Object)
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=== Q13 Abstract class vs interface ===
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Scooter has 2 wheels, charging, plug Type 2
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=== Q14 Polymorphism: what is and is not dynamic ===
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Child.who() | Parent.field | Parent.static | Child.static
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=== Q15 Can you override static, private or final methods? ===
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Base.secret | Base.locked
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=== Q16 How do you create an immutable class? ===
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members: [Asha, Ravi]
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add blocked: UnsupportedOperationException
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=== Q17 Composition vs inheritance (is-a vs has-a) ===
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stack = [99, 1, 2], pop gives 2
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=== Q18 Access modifiers ===
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a -> public
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b -> protected
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c -> (package-private)
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d -> private
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=== Q19 Object class methods: toString, equals, hashCode ===
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default toString looks like ClassName@hex: true
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p1.equals(p2) = true
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set contains an equal point: false
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=== Q20 Constructors: default, chained and private ===
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default config: dev:8080
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same Registry instance: true
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=== Q21 == vs equals() on Strings ===
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a == b -> true
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a == c -> false
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a.equals(c) -> true
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=== Q22 Why is String immutable? ===
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s = java
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upper = JAVA
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same object returned when nothing changes: true
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=== Q23 String vs StringBuilder vs StringBuffer ===
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builder result: cba-
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append is synchronized: StringBuffer=true, StringBuilder=false
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=== Q24 The string pool, compile-time constants and intern() ===
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"hel" + "lo" == "hello" -> true
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"hel" + lo == "hello" -> false
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"hel" + finalLo == "hello" -> true
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("hel" + lo).intern() == "hello" -> true
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=== Q25 Reverse a string and check for a palindrome ===
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weivretni
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true
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false
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=== Q26 Useful String methods and the split() surprises ===
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trim().length() = 6
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strip().length() = 2
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" ".isBlank() = true, "ab".repeat(3) = ababab
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"a,b,,".split(",").length = 2
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"a,b,,".split(",", -1).length = 4
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String.join: 2026-11-11
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=== Q27 Count characters, find the first non-repeating one, check anagrams ===
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frequency: {s=3, w=1, i=1}
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first non-repeating: w
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listen/silent anagrams: true
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=== Q28 The Collections Framework hierarchy ===
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ArrayList extends AbstractList implements [List, RandomAccess, Cloneable, Serializable]
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LinkedList extends AbstractSequentialList implements [List, Deque, Cloneable, Serializable]
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HashSet extends AbstractSet implements [Set, Cloneable, Serializable]
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TreeSet extends AbstractSet implements [NavigableSet, Cloneable, Serializable]
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ArrayDeque extends AbstractCollection implements [Deque, Cloneable, Serializable]
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HashMap extends AbstractMap implements [Map, Cloneable, Serializable]
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TreeMap extends AbstractMap implements [NavigableMap, Cloneable, Serializable]
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=== Q29 ArrayList vs LinkedList ===
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ArrayList is RandomAccess: true
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LinkedList is RandomAccess: false
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LinkedList is also a Deque: true
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after add(0, 5): [5, 10, 20, 30], get(2) = 20
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=== Q30 How HashMap works internally ===
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Aa: hashCode=2112, bucket (16 slots)=0
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BB: hashCode=2112, bucket (16 slots)=0
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Ankur: hashCode=63412443, bucket (16 slots)=12
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both survive a collision: first, second, size 2
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=== Q31 HashMap vs Hashtable vs ConcurrentHashMap vs TreeMap ===
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HashMap: null key accepted
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Hashtable: null key throws NullPointerException
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ConcurrentHashMap: null key throws NullPointerException
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TreeMap: null key throws NullPointerException
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=== Q32 HashSet vs LinkedHashSet vs TreeSet ===
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HashSet [banana, apple, cherry, pear, mango]
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LinkedHashSet [pear, apple, mango, banana, cherry]
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TreeSet [apple, banana, cherry, mango, pear]
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=== Q33 Comparable vs Comparator ===
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natural (Comparable): [Student[name=Asha, marks=88], Student[name=Meena, marks=88], Student[name=Ravi, marks=90]]
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marks desc, then name: [Student[name=Ravi, marks=90], Student[name=Asha, marks=88], Student[name=Meena, marks=88]]
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=== Q34 Fail-fast iterators and ConcurrentModificationException ===
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removing 2 inside for-each -> ConcurrentModificationException
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removing the second-to-last element threw nothing, list = [1, 2, 4]
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removeIf even -> [1, 3, 5]
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=== Q35 Arrays.asList() and List.of() pitfalls ===
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write-through to the array: changed
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Arrays.asList().add -> UnsupportedOperationException
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Arrays.asList(int[]) has 1 element
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List.of(.., null) -> NullPointerException
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=== Q36 Queue, Deque and PriorityQueue ===
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PriorityQueue polls: 1 2 3 4 5
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ArrayDeque as a stack pops: cba
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=== Q37 Mutable keys and the equals()/hashCode() contract in a HashMap ===
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get with the mutated key: null
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get with a fresh "A1" key: null
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size is still 1
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=== Q38 unmodifiableList() view vs List.copyOf() snapshot ===
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view sees the change: [a, b, c]
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copy does not: [a, b]
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=== Q39 Checked vs unchecked exceptions ===
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checked: NoSuchFileException (is an IOException: true)
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unchecked: ArithmeticException: / by zero
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NoSuchFileException is checked: true
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=== Q40 try, catch, finally and return ===
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finally runs before the method actually returns
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returned 1
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primitiveSnapshot = 10
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finallyOverrides = 7
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=== Q41 throw vs throws, and custom exceptions ===
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withdrawal refused <- cause: balance 100 < 500
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=== Q42 try-with-resources and suppressed exceptions ===
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open first
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open second
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close second
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close first
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caught: body failed
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suppressed: close failed: second
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=== Q43 Error vs Exception, and multi-catch ===
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Error, not Exception: Requested array size exceeds VM limit
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multi-catch caught ClassCastException
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=== Q44 Lambdas and functional interfaces ===
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180.0 true 6 hello 5
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=== Q45 Streams: map, filter, collect, laziness ===
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[ASHA, ARJUN]
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{4=2, 5=2}
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nothing visited yet: no terminal operation
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visited asha
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first = asha
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reuse -> stream has already been operated upon or closed
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=== Q46 Optional ===
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expensive() called
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orElse : value
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orElseGet : value
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nothing here
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=== Q47 Records ===
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Point[x=1, y=2] equals Point[x=1, y=2]: true, x() = 1
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rejected: negative coordinate
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=== Q48 var, switch expressions and text blocks ===
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{"name": "asha",
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"grade": "A"}
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=== Q49 Method references and default methods ===
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abc
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walk and swim
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=== Q50 Threads: start() vs run(), and virtual threads ===
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running on: main
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running on: worker-1
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virtual: true
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=== javac FinalOverride.java ===
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FinalOverride.java:6: error: locked() in Derived cannot override locked() in Base
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String locked() { return "Derived.locked"; }
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^
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overridden method is final
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1 error
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=== javac CheckedNotHandled.java ===
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CheckedNotHandled.java:6: error: unreported exception IOException; must be caught or declared to be thrown
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String s = Files.readString(Path.of("missing.txt"));
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^
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1 error
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=== javac LossyCast.java ===
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LossyCast.java:3: error: incompatible types: possible lossy conversion from double to int
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int x = 3.5;
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^
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LossyCast.java:4: error: incompatible types: possible lossy conversion from int to byte
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byte b = 200;
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^
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2 errors
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=== javac MissingReturnAndUnreachable.java ===
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MissingReturnAndUnreachable.java:4: error: missing return statement
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}
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^
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1 error
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=== javac --release 21 PreSuper.java ===
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PreSuper.java:8: error: flexible constructors is not supported in -source 21
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super();
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^
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(use -source 25 or higher to enable flexible constructors)
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1 error
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=== javac --release 25 PreSuper.java ===
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compiled without errors
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