JDK1.8增加了一个Predicate接口，现在就来探究一下。
后记
除了支持一个泛型参数的 Predicate 接口
还有接收对应类型的 DoublePredicate、IntPredicate、LongPredicate
还有接收两个泛型参数的 BiPredicate 接口
源码
/**
* Represents a predicate (boolean-valued function) of one argument.
*
* <p>This is a <a href="package-summary.html">functional interface</a>
* whose functional method is {@link #test(Object)}.
*
* @param <T> the type of the input to the predicate
*
* @since 1.8
*/
@FunctionalInterface
public interface Predicate<T> {
/**
* Evaluates this predicate on the given argument.
*
* @param t the input argument
* @return {@code true} if the input argument matches the predicate,
* otherwise {@code false}
*/
boolean test(T t);
/**
* Returns a composed predicate that represents a short-circuiting logical
* AND of this predicate and another. When evaluating the composed
* predicate, if this predicate is {@code false}, then the {@code other}
* predicate is not evaluated.
*
* <p>Any exceptions thrown during evaluation of either predicate are relayed
* to the caller; if evaluation of this predicate throws an exception, the
* {@code other} predicate will not be evaluated.
*
* @param other a predicate that will be logically-ANDed with this
* predicate
* @return a composed predicate that represents the short-circuiting logical
* AND of this predicate and the {@code other} predicate
* @throws NullPointerException if other is null
*/
default Predicate<T> and(Predicate<? super T> other) {
Objects.requireNonNull(other);
return (t) -> test(t) && other.test(t);
}
/**
* Returns a predicate that represents the logical negation of this
* predicate.
*
* @return a predicate that represents the logical negation of this
* predicate
*/
default Predicate<T> negate() {
return (t) -> !test(t);
}
/**
* Returns a composed predicate that represents a short-circuiting logical
* OR of this predicate and another. When evaluating the composed
* predicate, if this predicate is {@code true}, then the {@code other}
* predicate is not evaluated.
*
* <p>Any exceptions thrown during evaluation of either predicate are relayed
* to the caller; if evaluation of this predicate throws an exception, the
* {@code other} predicate will not be evaluated.
*
* @param other a predicate that will be logically-ORed with this
* predicate
* @return a composed predicate that represents the short-circuiting logical
* OR of this predicate and the {@code other} predicate
* @throws NullPointerException if other is null
*/
default Predicate<T> or(Predicate<? super T> other) {
Objects.requireNonNull(other);
return (t) -> test(t) || other.test(t);
}
/**
* Returns a predicate that tests if two arguments are equal according
* to {@link Objects#equals(Object, Object)}.
*
* @param <T> the type of arguments to the predicate
* @param targetRef the object reference with which to compare for equality,
* which may be {@code null}
* @return a predicate that tests if two arguments are equal according
* to {@link Objects#equals(Object, Object)}
*/
static <T> Predicate<T> isEqual(Object targetRef) {
return (null == targetRef)
? Objects::isNull
: object -> targetRef.equals(object);
}
}
翻译
/**
* 表示对输入参数进行断言（布尔值函数）
*
* 这是一个函数式接口，其函数式方法为test
*
* @param <T> 断言的输入类型
*
* @since 1.8
*/
@FunctionalInterface
public interface Predicate<T> {
/**
* 对给定参数进行断言求值
*
* @param t 输入参数
* @return 如果输入参数符合断言，则返回true，否则返回false
*/
boolean test(T t);
/**
* 返回一个由当前断言和other断言组成的短路逻辑与断言组合
* 当对断言组合求值时，如果当前断言返回false，则不执行other断言
*
* 任意断言执行过程中抛出的异常将被反馈到断言调用者
* 如果当前断言执行过程中抛出异常，这other断言不会被执行
*
* @param other 一个要和当前断言组合成逻辑与断言的断言
* @return 一个由当前断言和other断言组成的短路逻辑与断言组合
* @throws NullPointerException 如果other断言为null则抛出空指针异常
*/
default Predicate<T> and(Predicate<? super T> other) {
Objects.requireNonNull(other);
return (t) -> test(t) && other.test(t);
}
/**
* 返回一个和当前断言逻辑相反的断言
*
* @return 一个和当前断言逻辑相反的断言
*/
default Predicate<T> negate() {
return (t) -> !test(t);
}
/**
* 返回一个由当前断言和other断言组成的短路逻辑或断言组合
* 当对断言组合求值时，如果当前断言返回true，则不执行other断言
*
* 任意断言执行过程中抛出的异常将被反馈到断言调用者
* 如果当前断言执行过程中抛出异常，这other断言不会被执行
*
* @param other 一个要和当前断言组合成逻辑或断言的断言
* @return 一个由当前断言和other断言组成的短路逻辑或断言组合
* @throws NullPointerException 如果other断言为null则抛出空指针异常
*/
default Predicate<T> or(Predicate<? super T> other) {
Objects.requireNonNull(other);
return (t) -> test(t) || other.test(t);
}
/**
* 返回一个根据Objects#equals方法判断两个参数是否equal的断言
*
* @param <T> 断言的参数类型
* @param targetRef 用于比较相等的对象引用，有可能为null
* @return 一个根据Objects#equals方法判断两个参数是否equal的断言
*/
static <T> Predicate<T> isEqual(Object targetRef) {
return (null == targetRef)
? Objects::isNull
: object -> targetRef.equals(object);
}
}
摘要
- Predicate接口是函数式接口，其test方法用于返回一个boolean值
- Predicate接口的三个默认方法即与、或、非操作，静态方法为isEqual操作
实例
定义一个对象：
public class Student {
String name;
Sex sex;
int grade;
// 基础学费
double tuition = 10000.0;
public Student(String name, Sex sex, int grade) {
this.name = name;
this.sex = sex;
this.grade = grade;
}
enum Sex {
MALE, FEMALE
}
public void actualTuition(Student student, Predicate<Student> isPass, Consumer<Student> discount) {
// 一旦考试及格就可以学费打折
if (isPass.test(student)) {
discount.accept(student);
}
}
@Override
public String toString() {
return "Student [name=" + name + ", sex=" + sex + ", grade=" + grade + ", tuition=" + tuition + "]";
}
}
实例1，test、negate、isEqual：
Student s1 = new Student("lin", Sex.MALE, 80);
Student s2 = new Student("chen", Sex.FEMALE, 50);
Predicate<Student> grade = s -> s.grade > 60;
System.out.println("test1=" + grade.test(s1));
System.out.println("test2=" + grade.test(s2));
System.out.println("negate1=" + grade.negate().test(s1));
System.out.println("negate2=" + grade.negate().test(s2));
Predicate<Student> equal = Predicate.isEqual(s1);
System.out.println("isEqual1=" + equal.test(s1));
System.out.println("isEqual2=" + equal.test(s2));
输出结果为：
test1=true
test2=false
negate1=false
negate2=true
isEqual1=true
isEqual2=false
实例2，and、or：
Student s1 = new Student("lin", Sex.MALE, 80);
Student s2 = new Student("chen", Sex.FEMALE, 50);
Predicate<Student> grade = s -> s.grade > 60;
Predicate<Student> sex = s -> s.sex == Sex.FEMALE;
Predicate<Student> name = s -> s.name.startsWith("c");
Predicate<Student> and = grade.and(sex).and(name);
Predicate<Student> or = grade.or(sex).or(name);
System.out.println("and1=" + and.test(s1));
System.out.println("and2=" + and.test(s2));
System.out.println("or1=" + or.test(s1));
System.out.println("or2=" + or.test(s2));
输出结果为：
and1=false
and2=false
or1=true
or2=true
实例3，Predicate 和 Consumer：
Student s1 = new Student("lin", Sex.MALE, 80);
Student s2 = new Student("chen", Sex.FEMALE, 50);
System.out.println("计算前：" + s1.toString());
System.out.println("计算前：" + s2.toString());
Predicate<Student> isPass = s -> s.grade >= 60;
Consumer<Student> discount = s -> s.tuition = s.tuition * (100 - s.grade) / 100;
s1.actualTuition(s1, isPass, discount);
s2.actualTuition(s2, isPass, discount);
System.out.println("计算后：" + s1.toString());
System.out.println("计算后：" + s2.toString());
输出结果为：
计算前：Student [name=lin, sex=MALE, grade=80, tuition=10000.0]
计算前：Student [name=chen, sex=FEMALE, grade=50, tuition=10000.0]
计算后：Student [name=lin, sex=MALE, grade=80, tuition=2000.0]
计算后：Student [name=chen, sex=FEMALE, grade=50, tuition=10000.0]