從一段代碼看java對象初始化中屬性的初始化

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在java代碼中，當new一個對象的時候，先是建立了一個屬性為零值的對象，然後進行初始化:

```java
public class Test {
private Date date = new Date();
public static void main(String[] args) throws InterruptedException {
Test t = new Test();
for (int i = 0; i < 5; i++) {
Thread.sleep(1000);
System.out.println(t.date.toString());
}
}
}
```

該代碼運行結果:

```bash
Tue Jun 05 15:56:46 CST 2018
Tue Jun 05 15:56:46 CST 2018
Tue Jun 05 15:56:46 CST 2018
Tue Jun 05 15:56:46 CST 2018
Tue Jun 05 15:56:46 CST 2018
```

也就是說在建立Test類的對象t的時候，其date屬性被初始化成目前時間，後續再調用date屬性的時候會得到固定的值。而如果再次建立Test類的對象，則date屬性會發生變化:

```java
public class Test {
private Date date = new Date();
public static void main(String[] args) throws InterruptedException {
Test t = new Test();
for (int i = 0; i < 5; i++) {
System.out.println(t.date.toString());
}
System.out.println("--------------------------------------------");
Thread.sleep(1000);
t = new Test();
for (int i = 0; i < 5; i++) {
System.out.println(t.date.toString());
}
}
}
```

該代碼運行結果:

```bash
Tue Jun 05 16:02:25 CST 2018
Tue Jun 05 16:02:25 CST 2018
Tue Jun 05 16:02:25 CST 2018
Tue Jun 05 16:02:25 CST 2018
Tue Jun 05 16:02:25 CST 2018
--------------------------------------------
Tue Jun 05 16:02:26 CST 2018
Tue Jun 05 16:02:26 CST 2018
Tue Jun 05 16:02:26 CST 2018
Tue Jun 05 16:02:26 CST 2018
Tue Jun 05 16:02:26 CST 2018
```

而如果將date屬性設定為static的話，則即使再次建立Test類的對象， date屬性也不會發生變化:

```java
public class Test {
private static Date date = new Date();
public static void main(String[] args) throws InterruptedException {
Test t = new Test();
for (int i = 0; i < 5; i++) {
System.out.println(t.date.toString());
}
System.out.println("--------------------------------------------");
Thread.sleep(1000);
t = new Test();
for (int i = 0; i < 5; i++) {
System.out.println(t.date.toString());
}
}
}
```

該代碼運行結果:

```bash
Tue Jun 05 16:07:41 CST 2018
Tue Jun 05 16:07:41 CST 2018
Tue Jun 05 16:07:41 CST 2018
Tue Jun 05 16:07:41 CST 2018
Tue Jun 05 16:07:41 CST 2018
--------------------------------------------
Tue Jun 05 16:07:41 CST 2018
Tue Jun 05 16:07:41 CST 2018
Tue Jun 05 16:07:41 CST 2018
Tue Jun 05 16:07:41 CST 2018
Tue Jun 05 16:07:41 CST 2018
```

結論:

1. 針對同一個對象，其屬性會初始化一次，如果只是讀取這個屬性，這個屬性不會發生變化；
2. 如果建立一個類的不同執行個體，其屬性值有可能不同，而如果屬性值是static的話則是相同的。

從一段代碼看java對象初始化中屬性的初始化