Java编译器并不是简单的为每个引用都创建一个默认对象，如果想初始化这些引用可以在代码中的下列位置进行：
1.在定义对象得地方，这意味着他们总是能够被构造器调用之前被初始化。
2.在类的构造器中。
3.就在正要使用这些对象之前，这种方式称为惰性初始化，在生成对象不值得级不必每次生成对象的情况下，减少额外的负担。
4.使用实例初始化。

class Soap {
    private String s;

    Soap() {
        System.out.println("Soap()");
        s = "Constructed";
    }

    @Override
    public String toString() {
        return s;
    }
}

public class Bath {
    private String 
            s1 = "Happy", // 定义初始化
            s2 = "Happy",
            s3, s4;
    private Soap castile;
    private int i;
    private float toy;

    public Bath() {
        System.out.println("Inside Bath()");
        s3 = "Joy";
        toy = 3.14f;
        castile = new Soap(); // 在类的构造器中被初始化
    }

    {
        i = 47; // 实例初始化
    }

    @Override
    public String toString() {
        if (s4 == null) 
            s4 = "Joy"; // 惰性初始化
        return "s1 = " + s1 + "\n" +
                "s2 = " + s2 + "\n" +
                "s3 = " + s3 + "\n" +
                "s4 = " + s4 + "\n" +
                "i = " + i + "\n" +
                "toy = " + toy + "\n" +
                "castile = " + castile;
    }

    public static void main(String[] args) {
        Bath b = new Bath();
        System.out.println(b);
    }
} /* Output:
Inside Bath()
Soap()
s1 = Happy
s2 = Happy
s3 = Joy
s4 = Joy
i = 47
toy = 3.14
castile = Constructed
*///~