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法學研究Java 中非訪問修飾符 final、abstract、synchronized 是什麼？
abstract 關鍵字用於宣告抽象方法和抽象類。一旦一個方法被宣告為抽象方法，就不應該為其指定主體。並且一旦一個類被宣告為抽象類，則不能進行例項化。
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();
}
}
輸出
Constructing an Employee Call mailCheck using Employee reference-- Mailing a check to George W. Houston, TX
final 修飾符可以與方法、類和變數關聯。一旦宣告為 final，則不能例項化 final 類，不能覆蓋 final 方法，不能重新賦值 final 變數。
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();
}
}
輸出
10 6 Manager This is a final method
synchronized 關鍵字用於指示一次只能有一個執行緒訪問一個方法。synchronized 修飾符可以與任何這四個訪問級別修飾符一起使用。
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...");
}
}
}
}
}
輸出
Thread 1: Holding lock 1... Thread 2: Holding lock 2... Thread 1: Waiting for lock 2... Thread 2: Waiting for lock 1...
更新於：2019-07-30
327 次瀏覽
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