在重写java 1.4中的方法时如何避免“未使用的参数”警告？

Question

In this code :

public class MyClass {
    private Object innerValue;
    public Object getInnerValue() {
        return this.innerValue;
    }
    public void setInnerValue(Object innerValue) {
        this.innerValue = innerValue;
    }
}

public class MyClassReadOnly extends MyClass {
    MyClassReadOnly(MyClass cls) {
        // Make a field by field copy
        super.setInnerValue(cls.getInnerValue());
    }
    public void setInnerValue(Object innerValue) {
        throw new UnsupportedOperationException(
                            "This is a read-only instance"
                        );
    }
}

The compiler complains rightly about the unused parameter(never read) innerValue in MyClassReadOnly.setInnerValue() .

I don't want to disable this kind of warnings since it's quite useful usually, and I don't want to have any warnings either to have a high signal/noise ratio.

I cannot use the @SuppressWarnings() construct as another question suggested since it's Java 1.4 only.

I thought about inserting dummy code like this, but it's not very satisfactory :

public void setInnerValue(Object innerValue) {
    if (innerValue != null) { /* Do Nothing, but keep the compiler happy */ }
    throw new UnsupportedOperationException("This is a read-only instance");
}

Answer 1

The warning is not the problem, I'm afraid that the design is.

Your current hierarchy violates Liskov's principle of substitution since a class receiving an instance of a MyClass expects setInnerValue to work, and may not handle this exception correctly. You can say that a read-and-write X is a type of readable-X, but you cannot say that a readable-X is a type of read-and-writable X.

When I am faced with this sort of situation, I create an interface called IMyX with the reads, a subinterface called IMutableMyX with the writes, and then the actual class implements IMutableMyX and thus also IMyX. I am then very careful to only pass IMutableMyX when I need to, and pass IMyX in all other cases.

I feel that it is better to use the compiler and types to restrict access than it is to count on runtime exceptions. It also makes your code a lot clearer, and forces you to explicitly downcast the interface when you do want write-access.

I realize this does not answer your question about getting rid of the warnings. But warnings can either be suppressed, ignored, or addressed. An unused parameter is often a bad smell that indicates your method might not be doing what it's expected to do. Methods should only get essential parameters. If the parameter is not used, the parameter is not essential, and therefore something needs to be changed.

Answer 2

I wouldn't play any "code tricks" just to make a compiler warning go away, hoping for the compiler to optimize away the tricks. In fact, is this compiler warning all that useful? I would just disable it. Once you are using Java 5, you can use @SuppressWarnings and reenable it.

IMO, it's a bad idea to enable all possible warnings, just because they exist, and then set out to make every single warning go away. Figure out which warnings actually make sense for your environment and disable the rest.

Answer 3

I'm afraid you're stuck with dummy code. In C/C++, you could use a macro ( #define _unused(x) ((void) x) ), but (void) variable; isn't a valid statement in Java.

If it makes you feel better, the compiler will likely optimize away the empty if-block.

Answer 4

You can safely enter lines like: innerValue = null ; at the top of the function for all unused args. It will not affect the caller, but will keep the compiler happy.

Answer 5

If you're using Eclipse (?), you can turn on Parameter Is Never Read warnings, but ignore cases in overriding and implementing methods (which would solve this specific problem) and, separately, those documented with the "@param" tag (although of course that doesn't apply to Java 1.4). I would expect most other Java IDEs to have similar settings available.