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对于在运行时不需要测试的转换，编译器可能会进行一些优化以避免在运行时进行转换。
我建议阅读JLS Chapter 5. Conversions and Promotions以了解有关在运行时需要测试的转换类型的更多信息。
Example 5.0-1. Conversions at Compile-time and Run-time
A conversion from type Object to type Thread requires a run-time check to make sure that the run-time value is actually an instance of class Thread or one of its subclasses; if it is not, an exception is thrown.
A conversion from type Thread to type Object requires no run-time action; Thread is a subclass of Object, so any reference produced by an expression of type Thread is a valid reference value of type Object.
A conversion from type int to type long requires run-time sign-extension of a 32-bit integer value to the 64-bit long representation. No information is lost.
A conversion from type double to type long requires a nontrivial translation from a 64-bit floating-point value to the 64-bit integer representation. Depending on the actual run-time value, information may be lost.5.1.6. Narrowing Reference Conversion：
Such conversions require a test at run-time to find out whether the actual reference value is a legitimate value of the new type. If not, then a ClassCastException is thrown.
5.1.8. Unboxing Conversion;转换在运行时进行。
另见：5.5.3. Checked Casts at Run-time
例如，确定转换发生的时间并不容易：
public class Main {
private static class Child extends Parent{
public Child() {
}
}
private static class Parent {
public Parent() {
}
}
private static Child getChild() {
Parent o = new Child();
return (Child) o;
}
public static void main(final String[] args) {
Child c = getChild();
}
}javap -c Main给出的结果是：
public class Main extends java.lang.Object{
public Main();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."":()V
4: return
public static void main(java.lang.String[]);
Code:
0: invokestatic #4; //Method getChild:()LMain$Child;
3: astore_1
4: return
}如果将方法声明更改为public static Child getChild()，结果为：
public class Main extends java.lang.Object{
public Main();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."":()V
4: return
public static Main$Child getChild();
Code:
0: new #2; //class Main$Child
3: dup
4: invokespecial #3; //Method Main$Child."":()V
7: astore_0
8: aload_0
9: checkcast #2; //class Main$Child
12: areturn
public static void main(java.lang.String[]);
Code:
0: invokestatic #4; //Method getChild:()LMain$Child;
3: astore_1
4: return
}您看到只是更改访问者，可能会对可能的优化产生很大影响。
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