Getter and @Nonnull

Question

I get a warning from eclipse and I know I can remove it with suppress warning but I'd prefer to understand what makes it thing it could be null.

package-info.java

@ParametersAreNonnullByDefault
package test;

import javax.annotation.ParametersAreNonnullByDefault;

test.java

package test;


public class Test {
    public static void main( final String[ ] args ) {
        System.out.println( new Test( "a" ).getS( ) );
    }

    private final String s;

    public Test( final String s ) {
        this.s = s;
    }

    public String getS( ) {
        return this.s;//Null type safety: The expression of type String needs unchecked conversion to conform to '@Nonnull String'
    }
}

I don't get why I get this warning...

PS:

public Test( @Nonnull final String s ) { -> The nullness annotation is redundant with a default that applies to this location

@Nonnull private final String s; -> nothing changes

Answer 1

The problem is that the @Nonnull annotation has no effects on fields. It is only supported for:

• Method parameters
• Method return values
• Local variables (within a code block)

See eclipse Documentation

So quite obviously - as Nonnull is not checked on fields - the compiler doesn't know that Test.s is Nonnull and complains about it.

The obvious solution would indeed be to add a @SuppressWarnings("null") on the field access (or on the method, if it's a simple getter):

public String getS() {
    @SuppressWarnings("null")
    @Nonnull String s = this.s;
    return s;
}