引言
在 Java 8 中，stream (). Map ()允许您将一个对象转换为其他对象。查看下面例子：
1. 将 List 中的字符串转为大写
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. 将 List 中的对象转为字符串
public class Developer {
private String name;
private BigDecimal salary;
private Integer age;
//...
}
public static void main(String[] args) {
List<Developer> persons = Arrays.asList(
new Developer("zhangsan", 20),
new Developer("lisi",21),
new Developer("wangwu",22));
//Before Java 8
List<String> result = new ArrayList<>();
for (Developer developer : persons) {
result.add(developer.getName());
}
System.out.println(result); // [zhangsan, lisi, wangwu]
//Java 8
List<String> collect = persons.stream().map(x -> x.getName()).collect(Collectors.toList());
System.out.println(collect); // [zhangsan, lisi, wangwu]
}
3. 将 List 中的对象转为另一个对象
public class Person {
private String name;
private int age;
private String extra;
//...
}
- Java 8 之前：
List<Developer> developers = Arrays.asList(
new Developer("zhangsan", 20),
new Developer("lisi",21),
new Developer("wangwu",22));
List<Person> result = new ArrayList<>();
for (Developer developer : developers) {
Person person = new Person();
person.setName(developer.getName());
person.setAge(developer.getAge());
if ("lisi".equals(developer.getName())) {
person.setExtra("i am lisi");
}
result.add(person);
}
System.out.println(JSONUtil.toJsonStr(result));
}
- java 8
List<Developer> developers = Arrays.asList(
new Developer("zhangsan", 20),
new Developer("lisi",21),
new Developer("wangwu",22));
List<Person> result = developers.stream().map(temp -> {
Person person = new Person();
person.setName(temp.getName());
person.setAge(temp.getAge());
if ("lisi".equals(temp.getName())) {
person.setExtra("i am lisi");
}
return person;
}).collect(Collectors.toList());
System.out.println(JSONUtil.toJsonStr(result));
源码见：java-8-demo
系列文章详见：Java 8 教程
文档信息
- 本文作者：Zhang jinmiao
- 本文链接：http://zhangjinmiao.github.io/java8/2019/08/04/Java-8-Streams-map-%E4%BD%BF%E7%94%A8.html
- 版权声明：自由转载-非商用-非衍生-保持署名（创意共享3.0许可证）