集合框架

import java.util.ArrayList;
import java.util.List;
 
public class Main {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("Java");
        list.add("Python");
        list.add("Go");
        list.add(1, "TypeScript");   // 插入到索引 1
 
        System.out.println("size: " + list.size());
        System.out.println("get(0): " + list.get(0));
        System.out.println("contains Go? " + list.contains("Go"));
 
        for (String s : list) System.out.println("  " + s);
 
        list.remove("Python");
        System.out.println("remove 后: " + list);
    }
}

import java.util.HashMap;
import java.util.Map;
 
public class Main {
    public static void main(String[] args) {
        Map<String, Integer> ages = new HashMap<>();
        ages.put("Alice", 30);
        ages.put("Bob", 25);
        ages.put("Carol", 28);
 
        System.out.println("Alice: " + ages.get("Alice"));
        System.out.println("default: " + ages.getOrDefault("Dave", 0));
 
        ages.put("Alice", 31);   // 覆盖
        ages.putIfAbsent("Bob", 99);  // 不存在才放
        System.out.println("Alice: " + ages.get("Alice"));
        System.out.println("Bob: " + ages.get("Bob"));
 
        for (var e : ages.entrySet()) {
            System.out.printf("  %s -> %d%n", e.getKey(), e.getValue());
        }
    }
}

import java.util.HashSet;
import java.util.Set;
import java.util.TreeSet;
 
public class Main {
    public static void main(String[] args) {
        Set<Integer> s = new HashSet<>();
        s.add(3); s.add(1); s.add(3); s.add(2);
        System.out.println("HashSet: " + s);
 
        Set<Integer> ts = new TreeSet<>();
        ts.add(3); ts.add(1); ts.add(3); ts.add(2);
        System.out.println("TreeSet: " + ts);
    }
}

List<String> list = new ArrayList<>();   // 只能放 String
list.add(123);   // 编译错误

import java.util.*;
 
public class Main {
    public static void main(String[] args) {
        List<String> list = List.of("A", "B", "C");
 
        // 1. for-each
        for (String s : list) System.out.print(s + " ");
        System.out.println();
 
        // 2. Iterator
        Iterator<String> it = list.iterator();
        while (it.hasNext()) {
            System.out.print(it.next() + " ");
        }
        System.out.println();
 
        // 3. forEach + Lambda
        list.forEach(s -> System.out.print(s + " "));
        System.out.println();
    }
}

import java.util.*;
 
public class Main {
    static class Student {
        String name;
        Map<String, Integer> scores = new HashMap<>();
        Student(String name) { this.name = name; }
        void add(String course, int score) { scores.put(course, score); }
        double average() {
            return scores.values().stream()
                         .mapToInt(Integer::intValue)
                         .average()
                         .orElse(0);
        }
        @Override public String toString() {
            return String.format("%s 均分 %.1f", name, average());
        }
    }
 
    public static void main(String[] args) {
        Map<String, Student> db = new HashMap<>();
        db.put("Alice", new Student("Alice"));
        db.put("Bob", new Student("Bob"));
 
        db.get("Alice").add("语文", 85);
        db.get("Alice").add("数学", 92);
        db.get("Bob").add("语文", 78);
        db.get("Bob").add("数学", 88);
 
        for (var s : db.values()) System.out.println(s);
    }
}

// 任务：统计一句话中每个单词出现的次数
// 期望：Map<String, Integer>
// 输入："the quick brown fox jumps over the lazy dog the"
// 输出: {the=3, quick=1, brown=1, fox=1, jumps=1, over=1, lazy=1, dog=1}
 
import java.util.*;
 
public class Main {
    public static void main(String[] args) {
        String text = "the quick brown fox jumps over the lazy dog the";
        String[] words = text.split(" ");
        Map<String, Integer> freq = new HashMap<>();
        // 补全代码
        System.out.println(freq);
    }
}