# Java Program to Convert Infix Notation to PostFix Notation

Infix notation is the notation commonly used in arithmetical and logical formulae and statements. It is characterized by the placement of operators between operands – “infixed operators” – such as the plus sign in “2 + 2”.

Reverse Polish notation (RPN) or PostFix Notation is a mathematical notation in which every operator follows all of its operands such as the plus sign in “2 2 +”.

```import java.io.*;
import java.lang.*;
class stack {
private final int STACKSIZE = 50;
private int top;
private char items[];
public stack() {
top = -1;
items = new char[STACKSIZE];
}
public void push(char a) {
if (top == STACKSIZE - 1) {
System.out.println("overflow");
return;
} else
top++;
items[top] = a;
}
public boolean isEmpty() {
if (top == -1)
return true;
else
return false;
}
public char pop() {
if (top == -1) {
System.out.print("empty, cannot pop");
return ' ';
}
return (items[top--]);
}
public char peek() {
if (top == -1) {
System.out.println("empty, can not peek");
return ' ';
}
return (items[top]);
}
public void display() {
for (int i = top; i >= 0; i--)
System.out.println(items[i]);
}
}
class Infix2Postfix {
private boolean isOpenBracket(char m) {
if (m == '(')
return true;
else
return false;
}
private boolean isOperator(char m) {
if (m == '+' || m == '-' || m == '/' || m == '*')
return true;
else
return false;
}
private boolean isOperand(char m) {
if ((m >= '0' && m <= '9') || (m >= 'a' && m <= 'z') || (m >= 'A' && m <= 'Z'))
return true;
else
return false;
}
private int priority(char ch) {
int priority;
priority = 0;;
switch (ch) {
case '+':
priority = 3;
break;
case '-':
priority = 2;
break;
case '*':
priority = 5;
break;
case '/':
priority = 4;
break;
case '%':
priority = 1;
break;
}
return priority;
}
public void convert(String str) {
char ch;
stack s = new stack();
int i, j;
j = 0;
char postfix[] = new char[str.length()];
for (i = 0; i < str.length(); i++) {
ch = str.charAt(i);
if (isOpenBracket(ch))
s.push(ch);
else if (isOperand(ch))
postfix[j++] = ch;
else if (isOperator(ch))
if (s.isEmpty())
s.push(ch);
else {
if (priority(ch) >= priority(s.peek()))
s.push(ch);
else {
while (priority(ch) < priority(s.peek()))
postfix[j++] = s.pop();
s.push(ch);
}
}
else if (!isOpenBracket(ch)) {
while (!isOpenBracket(s.peek()))
postfix[j++] = s.pop();
if (isOpenBracket(s.peek()))
s.pop();
}
}
while (!s.isEmpty()) {
postfix[j++] = s.pop();
}
for (i = 0; i < str.length(); i++)
System.out.print(postfix[i]);
}
}
public class Infixtopostfix {
public static void main(String[] args) throws IOException {
String str;
Infix2Postfix in2post = new Infix2Postfix();
System.out.println();
System.out.println("Enter an expression in infix notation");
System.out.println("Postfix notation is");
in2post.convert(str);
}
}

/* Output
Enter an expression in infix notation
(A+B)*(C+D)

Postfix notation is
AB+CD+*

Enter an expression in infix notation
(A/B)*(C/D)
Postfix notation is
AB/CD/*

*/
```

## One thought on “Java Program to Convert Infix Notation to PostFix Notation”

1. Jherome says:

Thank you for sharing your knowledge, I really appreciate it.

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