HashMap中size-loadfactor-threshold-capacity

客官°小女子只卖身不卖艺 2022-05-21 23:36 130阅读 0赞

1.HashMap
    (1)size和capacity
    size:表示HashMap中已经装入多少个元素
    capacity表示HashMap中可以最多容纳多少个元素
    其中capacity默认的大小是16，设计成16主要是可以使用按位与替代取模来提升hash的效率。
    代码验证：
     Map<String,String> map = new HashMap<String, String>();
    map.put("test", "testValue");
    Class<?> mapType = map.getClass();
    Method capacity = null;
    try {
        capacity = mapType.getDeclaredMethod("capacity");
    } catch (NoSuchMethodException e) {
        e.printStackTrace();
    }
    capacity.setAccessible(true);
    try {
        System.out.println("capacity : " + capacity.invoke(map));
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    } catch (InvocationTargetException e) {
        e.printStackTrace();
    }
    
    Field size = null;
    try {
        size = mapType.getDeclaredField("size");
    } catch (NoSuchFieldException e) {
        e.printStackTrace();
    }
    size.setAccessible(true);
    try {
        System.out.println("size : " + size.get(map));
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    }
    (2)loadFactor和threshold
    loadFactor:负载因子，默认值为0.75
    threshold = capacity * loadFactor;
    当map所容纳的数量达到threshold的值时，hashMap就会自动扩容（JDK1.7是不能反映在capacity上，只有当容纳个数大于capacity时，才会反映在capacity上；
    但是实际中threshold的值已经发生改变，当容纳值大于12时。JDK1.8以上都会在容纳值大于12时，反映在capacity和threshold上）
    通过下面的代码对比可以发现，hashMap进行扩容应该是当量达到capacity的大小时才会进行扩容（代码所用JDK1.7）
    Map<String, String> map = new HashMap<String, String>();
    map.put("test1", "test");
    map.put("test2", "test");
    map.put("test3", "test");
    map.put("test4", "test");
    map.put("test5", "test");
    map.put("test6", "test");
    map.put("test7", "test");
    map.put("test8", "test");
    map.put("test9", "test");
    map.put("test10", "test");
    map.put("test11", "test");
    map.put("test12", "test");
    Class<?> mapType = map.getClass();
    
    Method capacity = mapType.getDeclaredMethod("capacity");
    capacity.setAccessible(true);
    System.out.println("capacity : " + capacity.invoke(map));
    
    Field size = mapType.getDeclaredField("size");
    size.setAccessible(true);
    System.out.println("size : " + size.get(map));
    
    Field threshold = mapType.getDeclaredField("threshold");
    threshold.setAccessible(true);
    System.out.println("threshold : " + threshold.get(map));
    
    Field loadFactor = mapType.getDeclaredField("loadFactor");
    loadFactor.setAccessible(true);
    System.out.println("loadFactor : " + loadFactor.get(map));
    
    结果：
    capacity : 16
    size : 12
    threshold : 12
    loadFactor : 0.75
    ===========================================================
    Map<String, String> map = new HashMap<String, String>();
    map.put("test1", "test");
    map.put("test2", "test");
    map.put("test3", "test");
    map.put("test4", "test");
    map.put("test5", "test");
    map.put("test6", "test");
    map.put("test7", "test");
    map.put("test8", "test");
    map.put("test9", "test");
    map.put("test10", "test");
    map.put("test11", "test");
    map.put("test12", "test");
    map.put("test13", "test");
    map.put("test14", "test");
    map.put("test15", "test");
    map.put("test16", "test");
    map.put("test17", "test");
    
    Class<?> mapType = map.getClass();
    
    Method capacity = mapType.getDeclaredMethod("capacity");
    capacity.setAccessible(true);
    System.out.println("capacity : " + capacity.invoke(map));
    
    Field size = mapType.getDeclaredField("size");
    size.setAccessible(true);
    System.out.println("size : " + size.get(map));
    
    Field threshold = mapType.getDeclaredField("threshold");
    threshold.setAccessible(true);
    System.out.println("threshold : " + threshold.get(map));
    
    Field loadFactor = mapType.getDeclaredField("loadFactor");
    loadFactor.setAccessible(true);
    System.out.println("loadFactor : " + loadFactor.get(map));
    
    结果：
    capacity : 32
    size : 17
    threshold : 24
    loadFactor : 0.75

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