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javabytecode

Understand ByteCode which print something

Please help what this bytecode will print

BIPUSH 10
BIPUSH 7
IXOR
ISTORE 1
IINC 1 19
GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
ILOAD 1
INVOKEVIRTUAL java/io/PrintStream.println (I)V
Solution

  • I would strongly recommend reading the JVM specification, which explains everything.

    Going through your specific example

    BIPUSH 10

    This pushes 10 onto the stack

    BIPUSH 7

    This pushes 7 onto the stack. The stack is now 10 7

    IXOR

    This xors the top two elements. The stack is now 10^7 = 13

    ISTORE 1

    This stores the top element in local variable slot 1. The stack is now empty, while the locals are [INVALID, 13]

    IINC 1 19

    This increments local 1 by 19. The locals are now [INVALID, 32]

    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;

    This pushes System.out onto the stack

    ILOAD 1

    This loads the variable onto the stack

    INVOKEVIRTUAL java/io/PrintStream.println (I)V

    And this prints it. So the final result is printing 32.

    This bytecode was probably generated by Java code along the lines of the following

    int x = 10 ^ 7;
x += 19;
System.out.println(x);