Java8 新特性 Streams map() 示例

2019年6月20日 224次阅读来源: chang_ge

在Java 8中stream().map()，您可以将对象转换为其他对象。查看以下示例：

1.大写字符串列表

1.1简单的Java示例将Strings列表转换为大写。

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.对象列表 – >字符串列表

2.1 name从staff对象列表中获取所有值。

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]

}

对象列表 – >其他对象列表

3.1此示例说明如何将staff对象列表转换为对象列表StaffPublic。

Staff.java
package com.mkyong.java8;

import java.math.BigDecimal;

public class Staff {

    private String name;
    private int age;
    private BigDecimal salary;
    //...
}

StaffPublic.java
package com.mkyong.java8;

public class StaffPublic {

    private String name;
    private int age;
    private String extra;
    //...
}

3.2 Java 8之前。

BeforeJava8.java

package com.mkyong.java8;

import java.math.BigDecimal;

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

public class BeforeJava8 {

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))

);

List<StaffPublic> result = convertToStaffPublic(staff);

System.out.println(result);

}

private static List<StaffPublic> convertToStaffPublic(List<Staff> staff) {

List<StaffPublic> result = new ArrayList<>();

for (Staff temp : staff) {

StaffPublic obj = new StaffPublic();

obj.setName(temp.getName());

obj.setAge(temp.getAge());

if ("mkyong".equals(temp.getName())) {

obj.setExtra("this field is for mkyong only!");

}

result.add(obj);

}

return result;

}

输出

[

StaffPublic{name='mkyong', age=30, extra='this field is for mkyong only!'},

StaffPublic{name='jack', age=27, extra='null'},

StaffPublic{name='lawrence', age=33, extra='null'}

]

3.3 Java 8的例子。

NowJava8.java

package com.mkyong.java8;

package com.hostingcompass.web.java8;

import java.math.BigDecimal;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class NowJava8 {

    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))
        );

        // convert inside the map() method directly.
        List<StaffPublic> result = staff.stream().map(temp -> {
            StaffPublic obj = new StaffPublic();
            obj.setName(temp.getName());
            obj.setAge(temp.getAge());
            if ("mkyong".equals(temp.getName())) {
                obj.setExtra("this field is for mkyong only!");
            }
            return obj;
        }).collect(Collectors.toList());

        System.out.println(result);

    }

}

输出

[
    StaffPublic{name='mkyong', age=30, extra='this field is for mkyong only!'},
    StaffPublic{name='jack', age=27, extra='null'},
    StaffPublic{name='lawrence', age=33, extra='null'}
]

    原文作者：chang_ge
    原文地址: https://blog.csdn.net/chang_ge/article/details/82957775
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