java - Lambda 表达式必须在方法内部吗？它们可以作为类的成员存在于方法之外吗

Question

我想看看在 Scala 和 Java 之间将函数作为参数传递的区别。几年前我搬到了 Scala，与 Java 接触不多。我可以在方法之外使用 Lambda 表达式吗，如下所示？它编译并工作正常，但它有什么问题吗？到目前为止，我看到的所有示例都在方法中包含了 Lamda。下面的代码只是一个例子，我知道你可以用 供应商界面。

@FunctionalInterface
public interface IFunc {

    String printMessage();
}


public class JavaFunc {

    public String message() {

        return functionA(functionB);
    }

    IFunc functionB = () ->  "Hi, I am functionB from Java, I am passed to functionA as a parameter.";

    public String functionA(IFunc funcB) {
        return funcB.printMessage();
    }

}

Answer 1

你可以在类的任何地方声明lambda表达式

If you declare inside a method (which is similar to local variables, scope inside of method)

public String message() {

  IFunc functionB = () ->  "Hi, I am functionB from Java, I am passed to functionA as a parameter.";

    return functionA(functionB);
}

If you declare in the class (Which is similar to instance variables that can be accessed in all instance methods directly and with object reference in static area )

public class JavaFunc {

 IFunc functionB = () ->  "Hi, I am functionB from Java, I am passed to functionA as a parameter.";

public String message() {

    // use here functionB
    return functionA(functionB);
}

示例: Predicate 和匿名内部类的 lambda 表达式

p1,p2 are at instance level and p3,p4 are local

public class DemoMain {

Predicate<Integer> p1 = i -> i > 5;

Predicate<Integer> p2 = new Predicate<Integer>() {

    @Override
    public boolean test(Integer t) {
        // TODO Auto-generated method stub
        return false;
    }
};

public static void main(String args[]) {

    DemoMain m = new DemoMain();
    System.out.println(m.p1.test(10));
    System.out.println(m.p2.test(10));
}

public void m1() {

    System.out.println(p1.test(10));
    System.out.println(p2.test(10));

    Predicate<Integer> p3 = i -> i > 5;

    Predicate<Integer> p4 = new Predicate<Integer>() {

        @Override
        public boolean test(Integer t) {
            // TODO Auto-generated method stub
            return false;
            }
        };

    }

 }