java中List常用的4种stream()方法解析

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;
public class Test {
public static void main(String[] args) {
Bean bean1 = new Bean(1, "a", "za");
Bean bean2 = new Bean(2, "b", "zb");
Bean bean3 = new Bean(3, "c", "zc");
ArrayList<Bean> list1 = new ArrayList();
list1.add(bean1);
list1.add(bean2);
list1.add(bean3);
//获取list1中的所有Bean对象的id，放入另一个list数组
//注意，这个.map()方法返回的还是流对象，而不是返回map的意思
//.stream().map().collect()
List<Long> ids = list1.stream().map(bean -> bean.getId()).collect(Collectors.toList());
System.out.println("ids=========" + ids);
//使用filter方法，筛选list1中的所有Bean对象，留下id属于ids中的Bean对象，并把它们的name字段转成一个list
//删除一个，便于看出差别来
ids.remove(0);
//.stream().filter().map().collect()
List<String> names = list1.stream().filter(bean -> ids.contains(bean.getId())).map(Bean::getName).collect(Collectors.toList());
//List<String> names = list1.stream().filter(bean -> ids.contains(bean.getId())).map(bean -> bean.getName()).collect(Collectors.toList());
System.out.println("names====" + names);
//把list1中的所有Bean对象转成map，其中，id是map的key，Bean对象本身是map的value
//.stream().collect()
Map<Long, Bean> maps = list1.stream().collect(java.util.stream.Collectors.toMap(Bean::getId, e -> e));
//Map<Long, String> maps = list1.stream().collect(Collectors.toMap(Bean::getId, e -> e.getName()));
System.out.println("maps====" + maps);
Set<Long> longs = maps.keySet();
System.out.println("longs====" + longs);
for (Long l : longs) {
System.out.print(l + " " + maps.get(l).getName() + " | ");
}
System.out.println();
//按条件过滤后生成map，与上方的filter类似，只保留ids中有的id的bean对象
//.stream().filter().collect()
Map<Long, Bean> maps2 = list1.stream().filter(bean -> ids.contains(bean.getId())).collect(java.util.stream.Collectors.toMap(Bean::getId, e -> e));
System.out.println(maps2);
Set<Long> longs2 = maps2.keySet();
for (Long l : longs2) {
System.out.print(l + " " + maps2.get(l).getName() + " | ");
}
System.out.println();
}
static class Bean {
private long id;
private String name;
private String address;
public Bean(long id, String name, String address) {
this.id = id;
this.name = name;
this.address = address;
}
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
}
}