问题描述
Java中Instance Initializers的含义是什么？
我们不能把这段代码放在构造函数的开头吗？
What is the sense of "Instance Initializers" in Java ?
Can't we just put that block of code at the beginning of the constructor instead?
推荐答案
我经常使用它们，通常用于在一个语句中创建和填充Map（而不是使用丑陋的静态块）：
I use them very often, typically for creating and populating Map in one statement (rather than using an ugly static block):
private static final Map<String, String> CODES = new HashMap<String, String>() {
{
put("A", "Alpha");
put("B", "Bravo");
}
};
一个有趣且有用的装饰是在一个地方创建一个不可修改的地图声明：
One interesting and useful embellishment to this is creating an unmodifiable map in one statement:
private static final Map<String, String> CODES =
Collections.unmodifiableMap(new HashMap<String, String>() {
{
put("A", "Alpha");
put("B", "Bravo");
}
});
比使用静态块和处理最终等的单数赋值方式更整洁。
Way neater than using static blocks and dealing with singular assignments to final etc.
另一个提示：不要害怕创建简化实例块的方法：
And another tip: don't be afraid to create methods too that simplify your instance block:
private static final Map<String, String> CODES = new HashMap<String, String>() {
{
put("Alpha");
put("Bravo");
}
void put(String code) {
put(code.substring(0, 1), code);
}
};
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