$ javap -p java.lang.IO
Compiled from "IO.java"
public final class java.lang.IO {
  private static java.io.BufferedReader br;
  private java.lang.IO();
  public static void println(java.lang.Object);
  public static void println();
  public static void print(java.lang.Object);
  public static java.lang.String readln();
  public static java.lang.String readln(java.lang.String);
  static synchronized java.io.BufferedReader reader();
}

$ javap -p -c java.lang.IO | grep -E 'public static|System.out|readLine|IOError'
  public static void println(java.lang.Object);
         0: getstatic     #14                 // Field java/lang/System.out:Ljava/io/PrintStream;
  public static void println();
         0: getstatic     #14                 // Field java/lang/System.out:Ljava/io/PrintStream;
  public static void print(java.lang.Object);
         0: getstatic     #14                 // Field java/lang/System.out:Ljava/io/PrintStream;
        10: invokevirtual #31                 // Method java/io/PrintStream.flush:()V
  public static java.lang.String readln();
         3: invokevirtual #40                 // Method java/io/BufferedReader.readLine:()Ljava/lang/String;
         8: new           #48                 // class java/io/IOError
        13: invokespecial #50                 // Method java/io/IOError."<init>":(Ljava/lang/Throwable;)V
  public static java.lang.String readln(java.lang.String);