Simple use of stream

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Preface

I don't understand a piece of code written by my colleagues , But I feel familiar ！

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One 、 Sample code

public class StreamTest{
    

    @Test
    public void contextLoads() {
    
        Map<String, Integer> items = new HashMap<>();
        items.put("a", 1);
        items.put("b", 2);
        items.put("c", 3);
        items.put("d", 4);
        items.put("e", 5);
        items.put("f", 6);

        for (Map.Entry<String, Integer> entry : items.entrySet()) {
    
 // System.out.println(entry);
            System.out.print(entry.getKey());
            System.out.println(entry.getValue());
        }
        // java8
        items.forEach((k, v) -> System.out.println(k + "==" + v));
        items.forEach((k, v) -> {
    
            System.out.print(k);
            System.out.println(v);
            if ("f".equals(k)) {
    
                System.out.println("hello f");
            }
        });
    }

    @Test
    public void Test02() {
    
        List<String> list = new ArrayList<>();
        list.add("a");
        list.add("b");
        list.add("c");
        list.add("d");
        list.add("e");
        list.add("f");
// for (String s : list) {
    
// System.out.print(s);// abcdef
// }
// for (int i = 0; i < list.size(); i++) {
    
// System.out.print(i);// 012345
// }
        // jdk8
        /** * lambda  expression  */
// list.forEach(s -> System.out.print(s)); // abcdef  It doesn't make much sense to use it alone 
// list.forEach(System.out::print); // abcdef  It doesn't make much sense to use it alone 
        list.forEach(str-> {
    
            if ("c".equals(str)){
    
                System.out.println("Hello c");
            }
        });
        /** * stream  Form of flow  */
        Stream<String> b = list.stream().filter(s -> s.contains("b"));
        System.out.println(b); // [email protected]
        Stream<String> stream = list.stream().filter(String::isEmpty);
        System.out.println(stream); // [email protected]
        System.err.println("===========");
        //  Meaning ： If the conditions are met, output , similar lambda+if sentence 
        list.stream().filter(str ->str.contains("b")).forEach(System.out::println); // b
        //  Use in work 
        List<User> userlist = new ArrayList<User>();
        //  Traverse List, Collect a number .  Commonly used in ： Subquery .  Batch modify order status 
        List<BigDecimal> ids = userlist.stream().map(user -> user.getId())
                .collect(Collectors.toList());
        List<String> names = userlist.stream().map(User::getName)
                .collect(Collectors.toList());

    }
}

class User{
    
    private BigDecimal id;
    private String name;

    public BigDecimal getId() {
    
        return id;
    }

    public void setId(BigDecimal id) {
    
        this.id = id;
    }

    public String getName() {
    
        return name;
    }

    public void setName(String name) {
    
        this.name = name;
    }
}

summary

1, Learn how to use stream Stream collection List Properties of objects in .

List<String> names = userlist.stream().map(User::getName)
                .collect(Collectors.toList());

2, To study the Collectors frequently-used 20 A way . link