java 中返回一个空Map

秒速五厘米 2024-03-04 06:12 182阅读 0赞

**原文链接：[Map用法总结][Map]**

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**Constructs an empty HashMap with the default initial capacity (16)** （`mutable`）

// Constructs an empty HashMap with the default initial capacity (16) and the default load fact
    // Since:1.2
    Map<String, String> map = new HashMap<>();
    map.put("key1", "value1");
    System.out.println(map); // {key1=value1}

**com.google.common.collect.Maps**（`mutable`）

// com.google.common.collect
    // Creates a mutable, empty HashMap instance.
    Map<String, String> map = Maps.newHashMap();
    map.put("key1", "value1");
    System.out.println(map); // {key1=value1}

**java.util.Collections**（`immutable`）

// Returns an empty map (immutable). This map is serializable.
    // Since: 1.5
    Map<String, String> map = Collections.emptyMap();
    map.put("key1", "value1");
    Exception in thread "main" java.lang.UnsupportedOperationException
    	at java.base/java.util.AbstractMap.put(AbstractMap.java:209)

**java.util.Collections**（`immutable`）

// The empty map (immutable). This map is serializable.
    // Since:1.3
    Map map = Collections.EMPTY_MAP;
    map.put("key1", "value1");
    Exception in thread "main" java.lang.UnsupportedOperationException
    	at java.base/java.util.AbstractMap.put(AbstractMap.java:209)

**org.apache.commons.collections.MapUtils**（`immutable`）

// An empty unmodifiable map.
    // This was not provided in JDK1.2.
    Map map = MapUtils.EMPTY_MAP;
    map.put("key1", "value1");
    Exception in thread "main" java.lang.UnsupportedOperationException
    	at org.apache.commons.collections.map.UnmodifiableMap.put(UnmodifiableMap.java:108)

[Map]: https://juejin.cn/post/7104985066605903908