Lambda Expression and Functional Interface in Java

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Java 8 引入了Lambda Expression的概念。lambda expression就是一个functional interface。

And a functional interface is an interface:

• With only one abstract method;
• Can define several Default methods;
• Can define several Static methods;
• Shall (not mandatory) be defined by annotation @FunctionalInterface

JAVA Compiler will use Function Interface method’s signature (parameter and return types) to check lambda expression.

有了Lambda expression之后，Java 7跟Java 8的代码就有了如下区别：

// Java 7
Predicate<String> p = new Predicate<String>() {
    public boolean test(String s) {
        return s.length() < 20;
    }
}

// Java 8
Predicate<String> p = s -> s.length() < 20;

Package java.util.function

There are 43 classes introduced in JDK 8, with four categories:

• Consumers
• Supplier
• Functions
• Predicates

Consumers

A consumer consumes an object, and doesn’t return anything.

public interface Consumer<T> {
    public void accept(T t);
}
public interface BiConsumer<T, V> {
    public void accept(T t, V v);
}

Suppliers

A supplier provides an object, take no parameter.

public interface Supplier<T> {
    public T get();
}

Functions

A function takes an object and returns another object.

public interface Function<T, R> {
    public R apply(T, t);
}
public interface BiFunction<T, V, R> {
    public R apply(T t, V v);
}
public interface UnaryOperator<T> extends Function<T, T> {}
public interface BinaryOperator<T> extends BiFunction<T, T, T> {}

Predicates

A predicate takes an object and return a boolean.

public interface Predicate<T> {
    public boolean test(T t);
}
public interface BiPredicate<T, U> {
    public boolean test(T t, U u);
}