Effective Java Item4:Enforce noninstantiability with a private constructor

Item4:Enforce noninstantiability with a private constructor

通过构造私有化，禁止对象被实例化。

public class UtilClass {

    private UtilClass(){
        //防止类内的函数调用构造函数创建对象
        throw new AssertionError();
    }

}