Java 8 Predicate 用法详解

本文使用简单的例子来说明 Predicate 的用法。

本文示例使用 Stream Api :

Stream<T> filter(Predicate<? super T> predicate);

使用 Predicate 来过滤 list :

例如我们有个数据 List 如下：

List<String> names = Arrays.asList("Jack", "Tom", "James", "Dwight Howard");

单条件过滤：

private static void listFilter(List<String> names) {
        List<String> result = names.stream()
                // 此处实际使用 Predicate
                .filter(name -> name.startsWith("J"))
                .collect(Collectors.toList());
        System.out.println("listFilter startsWith J :" + result);
    }

多条件过滤：

private static void listMultiFilter(List<String> names) {
        List<String> result = names.stream()
                // 此处实际使用 Predicate
                .filter(name -> name.startsWith("J"))
                .filter(name -> name.contains("c"))
                .collect(Collectors.toList());
        System.out.println("listMultiFilter 1 startsWith J and contains c :" + result);
    }

等同于

List<String> result = names.stream()
  // 此处实际使用 Predicate
  .filter(name -> name.startsWith("J") && name.contains("c"))
  .collect(Collectors.toList());

组合使用 Predicate：

Predicate.and() 两个条件都需要满足

private static void listFilterPredicateAnd(List<String> names) {
    Predicate<String> predicate1 = str -> str.contains("a");
    Predicate<String> predicate2 =  str -> str.length() > 5;
    List<String> result = names.stream()
            .filter(predicate1.and(predicate2))
            .collect(Collectors.toList());
    System.out.println("listFilterPredicateAnd contains a and length > 5 :" + result);
}

Predicate.or() 两个条件满足其中一个

private static void listFilterPredicateOr(List<String> names) {
    Predicate<String> predicate1 = str -> str.contains("a");
    Predicate<String> predicate2 =  str -> str.length() > 5;
    List<String> result = names.stream()
            .filter(predicate1.or(predicate2))
            .collect(Collectors.toList());
    System.out.println("listFilterPredicateOr contains a or length > 5 :" + result);
}

Predicate.negate() 对条件取反

private static void listFilterPredicateNegate(List<String> names) {
    Predicate<String> predicate1 = str -> str.contains("a");
    List<String> result = names.stream()
            .filter(predicate1.negate())
            .collect(Collectors.toList());
    System.out.println("listFilterPredicateNegate Does not contains a :" + result);
}

用 main 方法分别执行以上方法

public static void main(String[] args) {
    List<String> names = Arrays.asList("Jack", "Tom", "James", "Dwight Howard");
    listFilter(names);
    listMultiFilter(names);
    listFilterPredicateAnd(names);
    listFilterPredicateOr(names);
    listFilterPredicateNegate(names);
}

输出结果为:

listFilter startsWith J :[Jack, James]
listMultiFilter 1 startsWith J and contains c :[Jack]
listMultiFilter 2 startsWith J and contains c :[Jack]
listFilterPredicateAnd contains a and length > 5 :[Dwight Howard]
listFilterPredicateOr contains a or length > 5 :[Jack, James, Dwight Howard]
listFilterPredicateNegate Does not contains a :[Tom]

本文代码完整示例地址： Github