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法学研究Java 中非访问修饰符 final、abstract、synchronized 是什么？
abstract 关键字用于声明抽象方法和抽象类。一旦一个方法被声明为抽象方法，就不应该为其指定主体。并且一旦一个类被声明为抽象类，则不能进行实例化。
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
final 修饰符可以与方法、类和变量关联。一旦声明为 final，则不能实例化 final 类，不能覆盖 final 方法，不能重新赋值 final 变量。
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
synchronized 关键字用于指示一次只能有一个线程访问一个方法。synchronized 修饰符可以与任何这四个访问级别修饰符一起使用。
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：2019-07-30
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