积极求值与惰性求值
积极求值意味着表达式一遇到就求值,而惰性求值是指在需要时才求值。请参阅以下示例以了解这一概念。
import java.util.function.Supplier; public class FunctionTester { public static void main(String[] args) { String queryString = "password=test"; System.out.println(checkInEagerWay(hasName(queryString) , hasPassword(queryString))); System.out.println(checkInLazyWay(() -> hasName(queryString) , () -> hasPassword(queryString))); } private static boolean hasName(String queryString){ System.out.println("Checking name: "); return queryString.contains("name"); } private static boolean hasPassword(String queryString){ System.out.println("Checking password: "); return queryString.contains("password"); } private static String checkInEagerWay(boolean result1, boolean result2){ return (result1 && result2) ? "all conditions passed": "failed."; } private static String checkInLazyWay(Supplier<Boolean> result1, Supplier<Boolean> result2){ return (result1.get() && result2.get()) ? "all conditions passed": "failed."; } }
输出
Checking name: Checking password: failed. Checking name: failed.
此处 checkInEagerWay() 函数首先评估参数,然后执行其语句。而 checkInLazyWay() 则执行其语句并根据需要评估参数。由于 && 是短路运算符,checkInLazyWay 仅评估第一个参数(结果为 false),而根本不评估第二个参数。