public void swap(String a,String b)
{
String c=a;
a=b;
b=c;
}
如果不行就不行了！
Pass by Value
In Java methods, arguments are passed by value. When invoked, the method receives the value of the variable passed in. When the argument is of primitive type, pass-by-value means that the method cannot change its value. When the argument is of reference type, pass-by-value means that the method cannot change the object reference, but can invoke the object's methods and modify the accessible variables within the object.
This is often the source of confusion--a programmer writes a method that attempts to modify the value of one its arguments and the methoddo
esn't work as expected. Let's look at such method and then
investigate how to change it so that itdo
es what the programmer originally intended.
Consider this series of Java statements which attempts to retrieve the current color of a Pen object in a graphics application:
. . .
int r = -1, g = -1, b = -1;
pen.getRGBColor(r, g, b);
System.out.println("red = " + r +
", green = " + g +
", blue = " + b);
. . .
At the time when the getRGBColor method is called, the variables r, g, and b all have the value -1. The caller is expecting the getRGBColor method to pass back the red, green and blue values of the current color in the r, g, and b variables.
However, the Java runtime passes the variables' values (-1) into the getRGBColor method;
not a reference to the r, g, and b variables. So you could visualize the call to getRGBColor like this: getRGBColor(-1, -1, -1).
When control passes into the getRGBColor method, the arguments come into scope (get allocated) and are initialized to the value passed into the method:
class Pen {
int redValue, greenValue, blueValue;
void getRGBColor(int red, int green, int blue) {
// red, green, and blue have been created
// and their values are -1
. . .
}
}
So getRGBColor gets access to the values of r, g, and b in the caller through its arguments red, green, and blue, respectively. The method gets its own copy of the values to use within the scope of the method. Any changes made to those local copies are not reflected in the original variables from the caller.
Now, let's look at the implementation of getRGBColor within the Pen class that the method signature above implies:
class Pen {
int redValue, greenValue, blueValue;
. . .
// this methoddo
es not work as intended
void getRGBColor(int red, int green, int blue) {
red = redValue;
green = greenValue;
blue = blueValue;
}
}
This method will not work as intended. When control gets to the println statement in the following code, which was shown previously, getRGBColor's arguments, red, green, and blue, no longer exist. Therefore the assignments made to them within the method had no effect;
r, g, and b are all still equal to -1.
. . .
int r = -1, g = -1, b = -1;
pen.getRGBColor(r, g, b);
System.out.println("red = " + r +
", green = " + g +
", blue = " + b);
. . .
Passing variables by value affords the programmer some safety: Methods cannot unintentionally modify a variable that is outside of its scope. However, you often want a method to be able to modify one or more of its arguments. The getRGBColor method is a case in point. The caller wants the method to return three values through its arguments. However, the method cannot modify its arguments, and, furthermore, a method can only return one value through its return value. So, how can a method return more than one value, or have an effect (modify some value) outside of its scope?
For a method to modify an argument, it must be of a reference type such as an object or array. Objects and arrays are also passed by value, but the value of an object is a reference. So the effect is that arguments of reference types are passed in by reference. Hence the name. A reference to an object is the address of the object in memory. Now, the argument in the method is referring to the same memory location as the caller.
Let's rewrite the getRGBColor method so that it actuallydo
es what you want. First, you must introduce a new type of object, RGBColor, that can hold the red, green and blue values of a color in RGB space:
class RGBColor {
public int red, green, blue;
}
Now, we can rewrite getRGBColor so that it accepts an RGBColor object as an argument. The getRGBColor method returns the current color of the pen by setting the red, green and blue member variables of its RGBColor argument:
class Pen {
int redValue, greenValue, blueValue;
void getRGBColor(RGBColor aColor) {
aColor.red = redValue;
aColor.green = greenValue;
aColor.blue = blueValue;
}
}
And finally, let's rewrite the calling sequence:
. . .
RGBColor penColor = new RGBColor();
pen.getRGBColor(penColor);
System.out.println("red = " + penColor.red +
", green = " + penColor.green +
", blue = " + penColor.blue);
. . .
The modifications made to the RGBColor object within the getRGBColor method affect the object created in the calling sequence because the names penColor (in the calling sequence) and aColor (in the getRGBColor method) refer to the same object.
