visual basic barcode program 3: Operators and Assignments in Java

Painting PDF 417 in Java 3: Operators and Assignments

3: Operators and Assignments
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For this example, we ll use the Dimension class from the java.awt package:
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1. import java.awt.Dimension; 2. class ReferenceTest { 3. public static void main (String [] args) { 4. Dimension d = new Dimension(5,10); 5. ReferenceTest rt = new ReferenceTest(); 6. System.out.println("Before modify() d.height = " + d.height); 7. rt.modify(d); 8. System.out.println("After modify() d.height = " + d.height); 9. } 10. void modify(Dimension dim) { 11. dim.height = dim.height + 1; 12. System.out.println("dim = " + dim.height); 13. } 14. }
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When we run this class, we can see that the modify() method was indeed able to modify the original (and only) Dimension object created on line 4.
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C:\Java Projects\Reference>java ReferenceTest Before modify() d.height = 10 dim = 11 After modify() d.height = 11
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Notice when the Dimension object on line 4 is passed to the modify() method, any changes to the object that occur inside the method are being made to the object whose reference was passed. In the preceding example, reference variables d and dim both point to the same object.
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Does Java Use Pass-By-Value Semantics
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If Java passes objects by passing the reference variable instead, does that mean Java uses pass-by-reference for objects Not exactly, although you ll often hear and read that it does. Java is actually pass-by-value for all variables running within a single VM. Pass-by-value means pass-by-variable-value. And that means, pass-by-copy-of-the-variable ! It makes no difference if you re passing primitive or reference variables, you are always passing a copy of the bits in the variable. So for a primitive variable, you re passing a copy of the bits representing the value. For example, if you pass an int variable with the value of 3, you re passing a copy of the bits representing 3. The called method then gets its own copy of the value, to do with it what it likes.
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And if you re passing an object reference variable, you re passing a copy of the bits representing the reference to an object. The called method then gets its own copy of the reference variable, to do with it what it likes. But because two identical reference variables refer to the exact same object, if the called method modifies the object (by invoking setter methods, for example), the caller will see that the object the caller s original variable refers to has also been changed. In the next section, we ll look at how the picture changes when we re talking about primitives. The bottom line on pass-by-value: the called method can t change the caller s variable, although for object reference variables, the called method can change the object the variable referred to. What s the difference between changing the variable and changing the object For object references, it means the called method can t reassign the caller s original reference variable and make it refer to a different object, or null. For example, in the following code,
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void bar() { Foo f = new Foo(); doStuff(f); } void doStuff(Foo g) { g = new Foo(); }
reassigning g does not reassign f ! At the end of the bar() method, two Foo objects have been created, one referenced by the local variable f and one referenced by the local (argument variable) g. Because the doStuff() method has a copy of the reference variable, it has a way to get to the original Foo object, but the doStuff() method does not have a way to get to the f reference variable. So doStuff() can change what f refers to, but can t change the actual contents (bit pattern) of f.
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