Java Practices->Examine bytecode

Examine bytecode

Examination of the bytecode of a class is not a common task. However, if you're interested, it's possible to view bytecode using the javap tool included in the JDK. Here's an example of its use, where it shows the effect on bytecode of setting fields explicitly to their default initial values:

>javap -c -classpath . Quark
Compiled from Quark.java
public final class Quark extends java.lang.Object {
public Quark(java.lang.String,double);
}

Method Quark(java.lang.String,double)
   0 aload_0
   1 invokespecial #1 <Method java.lang.Object()>
   4 aload_0
   5 aconst_null
   6 putfield #2 <Field java.lang.String fName>
   9 aload_0
10 dconst_0
11 putfield #3 <Field double fMass>
14 aload_0
15 aload_1
16 putfield #2 <Field java.lang.String fName>
19 aload_0
20 dload_2
21 putfield #3 <Field double fMass>
24 return

Here is the source class itself :

public final class Quark {

  public Quark(String aName, double aMass){
    fName = aName;
    fMass = aMass;
  }

  //PRIVATE

  //WITHOUT redundant initialization to default values
  //private String fName;
  //private double fMass;

  //WITH redundant initialization to default values
  private String fName = null;
  private double fMass = 0.0d;
}

Bytecode instructions are defined by the JVM specification.