Is it right that this code
List<Integer> test2 = new ArrayList<Integer>();
test2.add(343);
int x2 = test2.get(0);
in compile time will be converted to this
List test = new ArrayList();
test.add(343);
int x = (Integer)test.get(0);
Something similar with autoboxing...
Let's try. Take this class with two methods that do absolutely the same thing with and without generics and autoboxing:
public class GenericTest{
// use generics and auto-boxing
// Java 1.5 or higher required
public void generic(){
final List<Integer> test2 = new ArrayList<Integer>();
test2.add(343);
final int x2 = test2.get(0);
}
// use neither generics nor auto-boxing,
// this should be Java 1.4-compatible
public void nonGeneric(){
final List test2 = new ArrayList();
test2.add(Integer.valueOf(343));
final int x2 = ((Integer) test2.get(0)).intValue();
}
}
Here's the byte code:
// Compiled from GenericTest.java (version 1.6 : 50.0, super bit)
public class GenericTest {
// Method descriptor #6 ()V
// Stack: 1, Locals: 1
public GenericTest();
0 aload_0 [this]
1 invokespecial java.lang.Object() [8]
4 return
Line numbers:
[pc: 0, line: 4]
Local variable table:
[pc: 0, pc: 5] local: this index: 0 type: GenericTest
// Method descriptor #6 ()V
// Stack: 2, Locals: 3
public void generic();
0 new java.util.ArrayList [16]
3 dup
4 invokespecial java.util.ArrayList() [18]
7 astore_1 [test2]
8 aload_1 [test2]
9 sipush 343
12 invokestatic java.lang.Integer.valueOf(int) : java.lang.Integer [19]
15 invokeinterface java.util.List.add(java.lang.Object) : boolean [25] [nargs: 2]
20 pop
21 aload_1 [test2]
22 iconst_0
23 invokeinterface java.util.List.get(int) : java.lang.Object [31] [nargs: 2]
28 checkcast java.lang.Integer [20]
31 invokevirtual java.lang.Integer.intValue() : int [35]
34 istore_2 [x2]
35 return
Line numbers:
[pc: 0, line: 7]
[pc: 8, line: 8]
[pc: 21, line: 9]
[pc: 35, line: 10]
Local variable table:
[pc: 0, pc: 36] local: this index: 0 type: GenericTest
[pc: 8, pc: 36] local: test2 index: 1 type: java.util.List
[pc: 35, pc: 36] local: x2 index: 2 type: int
Local variable type table:
[pc: 8, pc: 36] local: test2 index: 1 type: java.util.List<java.lang.Integer>
// Method descriptor #6 ()V
// Stack: 2, Locals: 3
public void nonGeneric();
0 new java.util.ArrayList [16]
3 dup
4 invokespecial java.util.ArrayList() [18]
7 astore_1 [test2]
8 aload_1 [test2]
9 sipush 343
12 invokestatic java.lang.Integer.valueOf(int) : java.lang.Integer [19]
15 invokeinterface java.util.List.add(java.lang.Object) : boolean [25] [nargs: 2]
20 pop
21 aload_1 [test2]
22 iconst_0
23 invokeinterface java.util.List.get(int) : java.lang.Object [31] [nargs: 2]
28 checkcast java.lang.Integer [20]
31 invokevirtual java.lang.Integer.intValue() : int [35]
34 istore_2 [x2]
35 return
Line numbers:
[pc: 0, line: 13]
[pc: 8, line: 14]
[pc: 21, line: 15]
[pc: 35, line: 16]
Local variable table:
[pc: 0, pc: 36] local: this index: 0 type: GenericTest
[pc: 8, pc: 36] local: test2 index: 1 type: java.util.List
[pc: 35, pc: 36] local: x2 index: 2 type: int
}
I don't see any obvious difference between the generic and the non-generic version, in fact here's the diff result for the two methods:
< public void generic();
---
> public void nonGeneric();
19,22c19,22
< [pc: 0, line: 7]
< [pc: 8, line: 8]
< [pc: 21, line: 9]
< [pc: 35, line: 10]
---
> [pc: 0, line: 13]
> [pc: 8, line: 14]
> [pc: 21, line: 15]
> [pc: 35, line: 16]
27,28d26
< Local variable type table:
< [pc: 8, pc: 36] local: test2 index: 1 type: java.util.List<java.lang.Integer>
As you can see, the only difference is in the line numbers and the local variable table.
Yes.
[[Mandatory filler to achieve > 30 chars]]
Yes:
List test = new ArrayList();
test.add(Integer.valueOf(343));
int x = ((Integer) test.get(0)).intValue();
