Why cast a String to a String?

Question

An article on the Oracle Java Community sites gives as an example (for a JPA Converter, but that's not relevant, I guess) the following method:

public Boolean convertToEntityAttribute(String y) {
    String val = (String) y;
    if(val.equals("Y")){
        return true;
    } else {
        return false;
    }
}

What is the use of casting the String y to a String val? Is there a valid reason to do this?

Original article: What's New in JPA

Mistake by a developer,like you and me. Thanks for Identifying :) — Abhiram mishra, Dec 07 '15 at 11:28
That whole sample code is awful. First of all, `AttributeConverter` needs two type parameters (as you'd expect, they are the two types you convert from/to), which the author neglects. As a result, the example doesn't even compile. Then there's the unnecessary cast and the icing on the cake is the `if (boolean) {return true;} else {return false;}` construct. — biziclop, Dec 07 '15 at 11:37
@farrellmr, though that's the better way of writing code, it will not give the same result as original code... So while **refactoring** you shouldn't do such changes without knowing the consequences. :) — Codebender, Dec 07 '15 at 15:08

score 13 · Accepted Answer · answered Dec 07 '15 at 11:27

Such cast is completely unnecessary. I can imagine that it was before

public Boolean convertToEntityAttribute(Object y) {
    String val = (String) y;
    ...
}

But later the argument type was changed to String and the author simply forgot to remove the cast.

Stephen C · Answer 2 · 2015-12-07T11:53:18.843

Is there a valid reason to do this?

None whatsoever¹.

But the flipside is that the Java compiler knows that the typecast is unnecessary and optimizes it away. So the only "damage" is to readability.

For example.

[stephen@blackbox tmp]$ cat Test.java 
public class Test {
    public void test (String x) {
        String s = (String) x;
        System.out.println(s);
    }
}
[stephen@blackbox tmp]$ javac Test.java 
[stephen@blackbox tmp]$ javap -c Test
Compiled from "Test.java"
public class Test {
  public Test();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public void test(java.lang.String);
    Code:
       0: aload_1
       1: astore_2
       2: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       5: aload_2
       6: invokevirtual #3                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       9: return
}
[stephen@blackbox tmp]$

The statement String s = (String) x; is compiled to a simple load and a store; no checkcast instruction.

I wouldn't be surprised if the JIT compiler was capable of optimizing away a redundant checkcast ... if it saw one.

^{1 - ... in hand written code. In source code that was generated, a redundant typecast could be serve the purpose of making it easier to write the source code generator. After all, the readability of generated code is largely irrelevant.}

score 2 · Answer 3 · answered Dec 07 '15 at 11:28

2

This redundant casting is useless.

Current code can be simplify to

public Boolean convertToEntityAttribute(String y) {
    return "Y".equals(y);
}

answered Dec 07 '15 at 11:28

Ruchira Gayan Ranaweera

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3

note that this is a different thing - code in the article would throw NPE on null values, whereas this would not. In some cases, NPE on null values is desired. – eis Dec 07 '15 at 11:30
@eis both cases would not cause NPE, you can cast null to any reference type without getting any exception http://stackoverflow.com/questions/18723596/no-exception-while-type-casting-with-a-null-in-java – Ian2thedv Dec 07 '15 at 11:33
1

@Ian2thedv but you certainly can't call val.equals() on null value without getting an exception – eis Dec 07 '15 at 11:34
@eis You will not get `NPE` by this. What are you referring ? – Ruchira Gayan Ranaweera Dec 07 '15 at 11:35
2

@RuchiraGayanRanaweera You will in the original code, so this is not equivalent to that. – biziclop Dec 07 '15 at 11:39

Why cast a String to a String?

3 Answers3