java.util.function中 Function, Supplier, Consumer, Predicate和其他函數(shù)式接口廣泛用在支持lambda表達(dá)式的API中。這些接口有一個(gè)抽象方法，會(huì)被lambda表達(dá)式的定義所覆蓋。

@FunctionalInterface
public interface Predicate<T> {

     boolean test(T t);

    default Predicate<T> and(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) && other.test(t);
    }

    default Predicate<T> negate() {
        return (t) -> !test(t);
    }

    default Predicate<T> or(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) || other.test(t);
    }

  static <T> Predicate<T> isEqual(Object targetRef) {
        return (null == targetRef)
                ? Objects::isNull
                : object -> targetRef.equals(object);
    }
}

Predicate接口主要用來(lái)判斷一個(gè)參數(shù)是否符合要求,類似于Junit的assert.

其核心方法如下:

• boolean test(T t);

  用來(lái)處理參數(shù)T,看起是否滿足要求,可以理解為  條件A
  

• default Predicate<T> and(Predicate<? super T> other),可理解為 條件A && 條件B

  調(diào)用當(dāng)前Predicate的test方法之后再去調(diào)用other的test方法,相當(dāng)于進(jìn)行兩次判斷

• default Predicate<T> negate()

  對(duì)當(dāng)前判斷進(jìn)行"!"操作,即取非操作，可理解為 ! 條件A

• default Predicate<T> or(Predicate<? super T> other)

  對(duì)當(dāng)前判斷進(jìn)行"||"操作,即取或操作,可以理解為 條件A ||條件B

• static <T> Predicate<T> isEqual(Object targetRef)

  對(duì)當(dāng)前操作進(jìn)行"="操作,即取等操作,可以理解為 A == B

下面對(duì)上述方法進(jìn)行實(shí)例測(cè)試：

test(T t)

判斷給定的值是否大于0

 Predicate<Integer> predicate = x -> x >  0;
 System.out.println(predicate.test(100));

結(jié)果為:

true

and(Predicate<? super T> other)

判斷給定的值是否是大于100的偶數(shù)

Predicate<Integer> predicate = x -> x >100;
predicate = predicate.and(x -> x % 2 == 0 );
System.out.println(predicate.test(98));
System.out.println(predicate.test(102));
System.out.println(predicate.test(103));

結(jié)果為:

false
true
false

negate()

計(jì)算一批用戶中年齡大于22歲的用戶的數(shù)量

Person類:

 class Person{
        private int age;
        private String name;
        public Person(int age, String name) {
            this.age = age;
            this.name = name;
        }
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        public int getAge() {
            return age;
        }
        public void setAge(int age) {
            this.age = age;
        }
    }

測(cè)試代碼:

Predicate<Person> personPredicate = x -> x.age > 22;
 System.out.println(
                Stream.of(
                        new Person(21,"zhangsan"),
                        new Person(22,"lisi"),
                        new Person(23,"wangwu"),
                        new Person(24,"wangwu"),
                        new Person(25,"lisi"),
                        new Person(26,"zhangsan")
                )
                        .filter(personPredicate.negate())
                        .count()
        );

結(jié)果為:

4

or(Predicate<? super T> other)

計(jì)算一批用戶中名稱為"lisi"或者年齡大于25歲的用戶的數(shù)量

 Predicate<Person> predicate =  x -> x.name.equals("lisi");
 predicate = predicate.or(x -> x.age > 25);
 System.out.println(
                Stream.of(
                        new Person(21,"zhangsan"),
                        new Person(22,"lisi"),
                        new Person(23,"wangwu"),
                        new Person(24,"wangwu"),
                        new Person(25,"lisi"),
                        new Person(26,"zhangsan")
                )
                        .filter(predicate)
                        .count()
        );

isEqual(Object targetRef)

假設(shè)認(rèn)為兩個(gè)用戶如果年齡一樣,名字一樣,我們認(rèn)為是一樣的,那我們來(lái)找下給定的一批用戶中一樣的用戶

Person類：

class Person{
        private int age;
        private String name;
        public Person(int age, String name) {
            this.age = age;
            this.name = name;
        }
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        public int getAge() {
            return age;
        }
        public void setAge(int age) {
            this.age = age;
        }
        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Person person = (Person) o;
            if (age != person.age ||!name.equals(person.name)) return false;
            return true;
        }
        @Override
        public int hashCode() {
            int result = age;
            result = 31 * result + name.hashCode();
            return result;
        }
    }

測(cè)試代碼:

Person person = new Person(22,"lisi");
Predicate<Person> predicate =  Predicate.isEqual(person);
System.out.println(
                Stream.of(
                        new Person(21,"zhangsan"),
                        new Person(22,"lisi"),
                        new Person(23,"wangwu"),
                        new Person(24,"wangwu"),
                        new Person(22,"lisi"),
                        new Person(26,"zhangsan")
                )
                        .filter(predicate)
                        .count()
        );

結(jié)果為：

2

與Predicate<T>相關(guān)的接口

• BiPredicate<T, U>

  針對(duì)兩個(gè)參數(shù),看兩個(gè)參數(shù)是否符合某個(gè)條件表達(dá)式

• DoublePredicate

  看一個(gè)double類型的值是否符合某個(gè)條件表達(dá)式

• IntPredicate

  看一個(gè)int類型的值是否符合某個(gè)條件表達(dá)式

• LongPredicate

  看一個(gè)long類型的值是否符合某個(gè)條件表達(dá)式