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Editorial Team
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Asked: 2026-05-27T18:14:05+00:00

Let’s say I have a Fraction class: class Fraction { … /** Invert current

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Let’s say I have a Fraction class:

class Fraction {
    ...

    /** Invert current fraction */
    public Fraction inverse() {
        return new Fraction(den,num);
    }

    ...
}

And this is what the bytecode of the above method turns out to be:

 0 new #1 <xyzTestes/system/fraction/Fraction>
 3 dup
 4 aload_0
 5 getfield #16 <xyzTestes/system/fraction/Fraction.den>
 8 aload_0
 9 getfield #14 <xyzTestes/system/fraction/Fraction.num>
12 invokespecial #27 <xyzTestes/system/fraction/Fraction.<init>>
15 areturn

I’m trying to understand why instruction at position 3 was put there in the first place. I’d say we’d only need to do the following to make it work:

 new #1 <xyzTestes/system/fraction/Fraction>
 aload_0
 getfield #16 <xyzTestes/system/fraction/Fraction.den>
 aload_0
 getfield #14 <xyzTestes/system/fraction/Fraction.num>
 invokespecial #27 <xyzTestes/system/fraction/Fraction.<init>>
 areturn

Why is not so?

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1 Answer

  1. Editorial Team

    When the bytecode for the constructor starts, there is no Fraction object. The new instruction allocates a Fraction object (uninitialized) from the heap and leaves a reference to it on the stack. The dup instruction is so that one reference can be used to call <init> and the second used for the areturn at the end.

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