[Java] Bad return type in lambda expression: int cannot be converted to boolean

If you are using functional interface using Lambda expression in your code and you get the below error, that's because your expression does not return the datatype which the Function Interface method expects,

package org.code2care;

@FunctionalInterface
public interface Validate {

    public boolean isString(Object object);

}

package org.code2care;

public class Example {

    public static void main(String[] args) {

        Validate validate = object -> object.toString();
        Validate validate1 = object -> object.toString().length();

        System.out.println(validate.isString("Java"));
    }
}

As you can see the reason for the complication error is isString(Object obj) returns a boolean and not java.lang.String or an int