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法律研究Java 中的 final、abstract、synchronized 非访问修饰符是什么？
关键词 abstract 用于声明抽象方法和抽象类。一旦某个方法被声明为抽象方法，我们就不应该为那些方法指定内容。一旦某个类被声明为抽象类，它将无法被实例化。
示例
abstract class Employee {
private String name;
private String address;
private int number;
public Employee(String name, String address, int number) {
System.out.println("Constructing an Employee");
this.name = name;
this.address = address;
this.number = number;
}
public double computePay() {
System.out.println("Inside Employee computePay");
return 0.0;
}
public void mailCheck() {
System.out.println("Mailing a check to " + this.name + " " + this.address);
}
public String toString() {
return name + " " + address + " " + number;
}
}
public class AbstractDemo extends Employee{
public AbstractDemo(String name, String address, int number) {
super(name, address, number);
}
public static void main(String [] args) {
Employee e = new AbstractDemo("George W.", "Houston, TX", 43);
System.out.println("
Call mailCheck using Employee reference--");
e.mailCheck();
}
}
输出
Constructing an Employee Call mailCheck using Employee reference-- Mailing a check to George W. Houston, TX
修饰符 final 可与方法，类和变量相关联。一旦声明最终版本，最终类将无法被实例化，最终方法将无法被覆盖，最终变量将无法被重新分配。
示例
class TestExample {
final int value = 10;
public static final int BOXWIDTH = 6;
static final String TITLE = "Manager";
public final void changeName() {
System.out.println("This is a final method");
}
}
final class Demo{ }
public class FinalExample extends TestExample {
public static void main(String args[]){
FinalExample obj = new FinalExample();
System.out.println(obj.value);
System.out.println(obj.BOXWIDTH);
System.out.println(obj.TITLE);
obj.changeName();
}
}
输出
10 6 Manager This is a final method
关键字 synchronized 用于指示方法只能由一个线程同时访问。同步修饰符可以与四种访问级别修饰符中的任何一种一起使用。
示例
public class TestThread {
public static Object Lock1 = new Object();
public static Object Lock2 = new Object();
public static void main(String args[]) {
ThreadDemo1 T1 = new ThreadDemo1();
ThreadDemo2 T2 = new ThreadDemo2();
T1.start();
T2.start();
}
private static class ThreadDemo1 extends Thread {
public void run() {
synchronized (Lock1) {
System.out.println("Thread 1: Holding lock 1...");
try { Thread.sleep(10); }
catch (InterruptedException e) {}
System.out.println("Thread 1: Waiting for lock 2...");
synchronized (Lock2) {
System.out.println("Thread 1: Holding lock 1 & 2...");
}
}
}
}
private static class ThreadDemo2 extends Thread {
public void run() {
synchronized (Lock2) {
System.out.println("Thread 2: Holding lock 2...");
try { Thread.sleep(10); }
catch (InterruptedException e) {}
System.out.println("Thread 2: Waiting for lock 1...");
synchronized (Lock1) {
System.out.println("Thread 2: Holding lock 1 & 2...");
}
}
}
}
}
输出
Thread 1: Holding lock 1... Thread 2: Holding lock 2... Thread 1: Waiting for lock 2... Thread 2: Waiting for lock 1...
更新于: 30-7-2019
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