Java 8 Streams map() 例子__Java

在java 8，stream().map()允許您將對象轉換為別的。複習下面的例子：
1. String類型的List集合轉大寫 

1.1 簡單的java例子把字串列錶轉換為大寫的情況. TestJava8.java

package com.mkyong.java8;import java.util.ArrayList;import java.util.Arrays;import java.util.List;import java.util.stream.Collectors;public class TestJava8 {    public static void main(String[] args) {        List<String> alpha = Arrays.asList("a", "b", "c", "d");        //Before Java8        List<String> alphaUpper = new ArrayList<>();        for (String s : alpha) {            alphaUpper.add(s.toUpperCase());        }        System.out.println(alpha); //[a, b, c, d]        System.out.println(alphaUpper); //[A, B, C, D]        // Java 8        List<String> collect = alpha.stream().map(String::toUpperCase).collect(Collectors.toList());        System.out.println(collect); //[A, B, C, D]        // Extra, streams apply to any data type.        List<Integer> num = Arrays.asList(1,2,3,4,5);        List<Integer> collect1 = num.stream().map(n -> n * 2).collect(Collectors.toList());        System.out.println(collect1); //[2, 4, 6, 8, 10]    }}

2. List of objects -> List of String

2.1 從 staff 對象集合中擷取所有的 name 值. Staff.java

package com.mkyong.java8;import java.math.BigDecimal;public class Staff {    private String name;    private int age;    private BigDecimal salary;//...}

TestJava8.java

package com.mkyong.java8;import java.math.BigDecimal;import java.util.ArrayList;import java.util.Arrays;import java.util.List;import java.util.stream.Collectors;public class TestJava8 {    public static void main(String[] args) {        List<Staff> staff = Arrays.asList(                new Staff("mkyong", 30, new BigDecimal(10000)),                new Staff("jack", 27, new BigDecimal(20000)),                new Staff("lawrence", 33, new BigDecimal(30000))        );        //Before Java 8        List<String> result = new ArrayList<>();        for (Staff x : staff) {            result.add(x.getName());        }        System.out.println(result); //[mkyong, jack, lawrence]        //Java 8        List<String> collect = staff.stream().map(x -> x.getName()).collect(Collectors.toList());        System.out.println(collect); //[mkyong, jack, lawrence]    }}

3. List of objects -> List of other objects

3.1  這個例子展示如何將 staff  對象的集合轉為 StaffPublic 對象的集合. Staff.java

package com.mkyong.java8