Eclipse – easily looking at Java bytecode

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Eclipse – easily looking at Java bytecode

December 26th, 2014 by Jeanne Boyarsky

A fellow moderator asked me to weigh in on this question at CodeRanch. The gist is whether this code creates one String or two:

String s = " " + 3;

How to find out the answer

The most definitive way to verify this is to check the bytecode. I had downloaded the bytecode plugin when working on our Java 8 OCA Study Guide because sometimes you just have to know what actually goes on behind the scenes to be accurate.

Using the plugin is easy. You go to Window -> Show View -> Other -> Java -> Bytecode. Then every time you save the Java file, the bytecode window is automatically updated. Great for lots of iterations.

The test

I wrote a simple Java class:

package jb;
public class PlayTest {
  public static void main(String[] args) {
    String s = "" + 3;
  }
}

The generated bytecode is:

// class version 52.0 (52)
// access flags 0x21
public class jb/PlayTest {

  // compiled from: PlayTest.java

  // access flags 0x1
  public <init>()V
   L0
    LINENUMBER 4 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.<init> ()V
    RETURN
   L1
    LOCALVARIABLE this Ljb/PlayTest; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1

  // access flags 0x9
  public static main([Ljava/lang/String;)V
   L0
    LINENUMBER 8 L0
    LDC "3"
    ASTORE 1
   L1
    LINENUMBER 14 L1
    RETURN
   L2
    LOCALVARIABLE args [Ljava/lang/String; L0 L2 0
    LOCALVARIABLE s Ljava/lang/String; L1 L2 1
    MAXSTACK = 1
    MAXLOCALS = 2
}

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