java学习笔记（三）java中的修饰符abstract、static与final

package wbb.java.modifies;

/**
 * Created with IntelliJ IDEA.
 * User: wbb
 * Date: 14-7-17
 * Time: 下午2:52
 * To change this template use File | Settings | File Templates.
 */
public abstract class AbstractDemo {
    public abstract void xxx();
}

abstract class Test extends AbstractDemo {
//    public void xxx(){}   //用abstract修饰子类，或者重写父类中的抽象方法
}

package wbb.java.modifies;

/**
 * Created with IntelliJ IDEA.
 * User: wbb
 * Date: 14-7-17
 * Time: 下午3:45
 * To change this template use File | Settings | File Templates.
 */
public class StaticDemo {
    public static final String sqlStr = "SELECT * FROM DUAL";
}

class A extends StaticDemo{
    StaticDemo a = new StaticDemo();
    public static void main(String[] args) {
        System.out.println(StaticDemo.sqlStr);
    }
}

一般情况下,如果有些代码必须在项目启动的时候就执行的时候,需要使用静态代码块,这种代码是主动执行的;需要在项目启动的时候就初始化,在不创建对象的情况下,其他程序来调用的时候,需要使用静态方法,这种代码是被动执行的.静态代码块的作用可用于初始化类，给类的成员变量赋值等等。

package wbb.java.modifies;

/**
 * Created with IntelliJ IDEA.
 * User: wbb
 * Date: 14-7-17
 * Time: 下午4:04
 * To change this template use File | Settings | File Templates.
 */
public class StaticProgram {
    static {
        System.out.println("静态代码块执行");
    }

    public static void main(String[] args) {
        //运行结果：输出“静态代码块执行”
    }
}

四、final修饰符

package wbb.java.modifies;

/**
 * Created with IntelliJ IDEA.
 * User: wbb
 * Date: 14-7-17
 * Time: 下午4:10
 * To change this template use File | Settings | File Templates.
 */
public final class FinalDemo {
    private static final int num = 1;

    public static void main(String[] args) {
//        num ++; 被final修饰的变量不能改变
    }
}

//class TestFinalClass extends FinalDemo {} 被final修饰的类不能被继承

class TestFinalFun {
    public final void finalFun() { }
}

class TestFinalFun2 extends TestFinalFun {
//    public void finalFun() { }  被final修饰的方法不能被重写
}