Java Collection Lambda表达式遍历

2024/3/11 Java 笔记 Collection Lambda

Java的Collection可以通过迭代器、增强for、forEach遍历，而forEach遍历中又可以使用lambda表达式简化代码，现在从源码上分析Lambda表达式遍历的原理。

正常使用forEach遍历示例

Collection<String> col = new ArrayList<>();
col.forEach(new Consumer<String>(){
    @Override
    //s依次表示集合中每一个元素
    public void accept(String s) {
        System.out.println(s);
    }
});

forEach()源码

default void forEach(Consumer<? super T> action) {
        Objects.requireNonNull(action);
        for (T t : this) {
            action.accept(t);
        }
    }

可以看到是参数是一个Consumer<? super T> action，之后使用增强for遍历了集合内部的元素

Consumer<? super T>源码

@FunctionalInterface
public interface Consumer<T> {

    /**
     * Performs this operation on the given argument.
     *
     * @param t the input argument
     */
    void accept(T t);

    /**
     * Returns a composed {@code Consumer} that performs, in sequence, this
     * operation followed by the {@code after} operation. If performing either
     * operation throws an exception, it is relayed to the caller of the
     * composed operation.  If performing this operation throws an exception,
     * the {@code after} operation will not be performed.
     *
     * @param after the operation to perform after this operation
     * @return a composed {@code Consumer} that performs in sequence this
     * operation followed by the {@code after} operation
     * @throws NullPointerException if {@code after} is null
     */
    default Consumer<T> andThen(Consumer<? super T> after) {
        Objects.requireNonNull(after);
        return (T t) -> { accept(t); after.accept(t); };
    }
}

可以看到是一个接口，并且标注了 @FunctionalInterface，可以用lambda表达式来充当这个接口的实现类

使用lambda后的代码

col.forEach(s -> System.out.println(s));

可以发现大大简化了
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