常见Java集合的实现细节(一)：Map和Set的关系

import java.io.Serializable;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;

class SimpleEntry<K, V> implements Entry<K, V>, Serializable {
    
    private final K key;
    private V value;
    
    public SimpleEntry(K key, V value) {
        this.key = key;
        this.value = value;
    }
    
    public SimpleEntry(Entry<? extends K, ? extends V> entry) {
        this.key = entry.getKey();
        this.value = entry.getValue();
    }

    @Override
    public K getKey() {
        return key;
    }

    @Override
    public V getValue() {
        return value;
    }

    /**
     * 改变 key-value 对的 value 值
     */
    @Override
    public V setValue(V value) {
        V oldValue = this.value;
        this.value = value;
        return oldValue;
    }

    @Override
    public int hashCode() {
        return key == null ? 0 : key.hashCode();
    }

    @Override
    public boolean equals(Object obj) {
        if(obj == this) {
            return true;
        }
        if(obj.getClass() == this.getClass()) {
            SimpleEntry that = (SimpleEntry) obj;
            return that.getKey().equals(this.getKey());
        }
        return false;
    }

    @Override
    public String toString() {
        return this.key + "=" + this.value;
    }
    
}

public class Set2Map<K, V> extends HashSet<SimpleEntry<K, V>> {
    
    // 清空 Set
    public void clean() {
        super.clear();
    }
    
    // 判断是否包含 key
    public boolean containsKey(K key) {
        return super.contains(new SimpleEntry<K, V>(key, null));
    }
    
    // 判断是否包含 value
    public boolean containsValue(V value) {
        for(SimpleEntry<K, V> entry : this) {
            if(entry.getValue().equals(value)) {
                return true;
            }
        }
        return false;
    }
    
    // 根据指定 key 取出对应的 value
    public V get(K key) {
        for(SimpleEntry<K, V> entry : this) {
            if(entry.getKey().equals(key)) {
                return entry.getValue();
            }
        }
        return null;
    }
    
    // 将指定 key-value 对放入Set中
    public V put(K key, V value) {
        add(new SimpleEntry<K, V>(key, value));
        return value;
    }
    
    // 将另一个 Map 的 key-value 对放入本 Map 中
    public void putAll(Map<? extends K, ? extends V> map) {
        for(K key : map.keySet()) {
            add(new SimpleEntry<K, V>(key, map.get(key)));
        }
    }
    
    // 根据指定的 key 删除 key-value 对
    public V removeEntry(K key) {
        Iterator<SimpleEntry<K, V>> iterator = this.iterator();
        while(iterator.hasNext()) {
            SimpleEntry<K, V> entry = iterator.next();
            if(entry.getKey().equals(key)) {
                V value = entry.getValue();
                iterator.remove();
                return value;
            }
        }
        return null;
    }
    
    public int size() {
        return super.size();
    }
    
}

public class Set2MapTest {

    public static void main(String[] args) {
        
        Set2Map<String, Integer> scores = new Set2Map<>();

        scores.put("语文", 89);
        scores.put("数学", 83);
        scores.put("英文", 80);
        System.out.println(scores);
        System.out.println(scores.size());
        scores.removeEntry("数学");
        System.out.println(scores);
        System.out.println(scores.get("语文"));
        System.out.println(scores.containsKey("英文"));
        System.out.println(scores.containsValue(89));
        scores.clean();
        System.out.println(scores);
        
    }
    
}

运行结果：
[英文=80, 数学=83, 语文=89]
3
[英文=80, 语文=89]
89
true
true
[]