Java8 新特性 Streams map() 示例

在Java 8中

stream().map()

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

staff

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

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'}
]