Detailed explanation of Java initialization method classes and containers

Java initialization method: class, container

Initialization class (non-final):

In the DefaultActionMapper class of Struts2:

public DefaultActionMapper() {
    prefixTrie = new PrefixTrie() { {
        put(METHOD_PREFIX, new ParameterAction() {
            public void execute(String key, ActionMapping mapping) {
                if (allowDynamicMethodCalls) {
                    mapping.setMethod(key.substring( METHOD_PREFIX.length()));
                }
            }
         }); //。。。。
     }};
}

put is the method of PrefixTrie: public void put(String prefix, Object value) ;

Initialize container:

Original method in collection framework (collections, such as list, map, set, etc.):

Set<String> myset = new HashSet<String>();
myset.add("aa");
myset.add("bb");
myset.add("cc");
myset.add("dd");
domethod(myset);

Static initialization Method:

private static final Set<String> myset = new HashSet<String>();
static { myset.add("aa");
    myset.add("bb");
    myset.add("cc");
    myset.add("dd");
}

Double-brace syntax (double-brace syntax) creates and initializes a new collection:

private static final Set<String> myset = new HashSet<String>() {{
    add("aa");
    add("bb");
    add("cc");
    add("dd");
}};

doMethod(new HashSet<String>() {{
    add("aa");
    add("bb");
    add("cc");
    add("dd");
}});

First level of brackets It actually defines an internal anonymous class (Anonymous Inner Class);

The second layer of brackets is actually an instance initialization block ( instance initializer block), this block is executed during the construction of the inner anonymous class.

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