The Predicate interface is used primarily to filter a Stream object. Using the filter() method with the Predicate interface parameter will allow us to filter the items in the Stream object to satisfy some conditions.
The content of this interface is as follows:
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/* * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.util.function; import java.util.Objects; /** * 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); } /** * Returns a predicate that is the negation of the supplied predicate. * This is accomplished by returning result of the calling * {@code target.negate()}. * * @param <T> the type of arguments to the specified predicate * @param target predicate to negate * * @return a predicate that negates the results of the supplied * predicate * * @throws NullPointerException if target is null * * @since 11 */ @SuppressWarnings("unchecked") static <T> Predicate<T> not(Predicate<? super T> target) { Objects.requireNonNull(target); return (Predicate<T>)target.negate(); } } |
As a Functional Interface, the Predicate has an abstract method test(). This method takes the parameter with the generic data type, will return true or false to indicate whether the item satisfies the filter condition.
Eg:
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package com.huongdanjava.javaexample; import java.util.Arrays; import java.util.List; public class Example { public static void main(String[] args) { List<String> name = Arrays.asList("Khanh", "Quan"); name.stream() .filter(s -> s.startsWith("K")) .forEach(System.out::println); } } |
In the above example, you can see that Java inspects all items in the Stream object for items that start with K, and then prints them.
Result:
In the Predicate interface we also have some default methods such as and(), or(), negate() with functions like and-and, or, negative.
With the and() method, we can aggregate many conditions to filter the data. Eg:
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package com.huongdanjava.javaexample; import java.util.Arrays; import java.util.List; import java.util.function.Predicate; public class Example { public static void main(String[] args) { List<String> name = Arrays.asList("Khanh", "Quan", "Khang"); Predicate<String> p1 = s -> s.startsWith("K"); Predicate<String> p2 = s -> s.endsWith("g"); name.stream() .filter(p1.and(p2)) .forEach(System.out::println); } } |
Result:
The or() method only satisfies one of the conditions. Eg:
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package com.huongdanjava.javaexample; import java.util.Arrays; import java.util.List; import java.util.function.Predicate; public class Example { public static void main(String[] args) { List<String> name = Arrays.asList("Khanh", "Quan", "Khang"); Predicate<String> p1 = s -> s.startsWith("K"); Predicate<String> p2 = s -> s.endsWith("g"); name.stream() .filter(p1.or(p2)) .forEach(System.out::println); } } |
Result:
The negate() method negates the condition. Eg:
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package com.huongdanjava.javaexample; import java.util.Arrays; import java.util.List; import java.util.function.Predicate; public class Example { public static void main(String[] args) { List<String> name = Arrays.asList("Khanh", "Quan", "Khang"); Predicate<String> p1 = s -> s.startsWith("K"); name.stream() .filter(p1.negate()) .forEach(System.out::println); } } |
Result:
