| << Chapter < Page | Chapter >> Page > |
Java programmers make mistakes in the use of the assignment and equality operators, especially when strings are used. The concept of referencesemantics is (or should be) explained in detail in your textbook, so we limit ourselves to a reminder of the potential problems.
Given:
String s1 = "abcdef";
String s2 = "abcdef";String s3 = "abc" + "def";
String s4 = "abcdef" + "";String s5 = s1 + "";
String t1 = "abc";String t2 = "def";
String s6 = t1 + t2;
all strings
sn are equal when compared using the method
equals in the class
String that compares the
contents pointed to by the
reference:
if (s1.equals(s5)) // Condition evaluates to true
The string
literal
"abcdef" is stored only once, so strings
s1 ,
s2 and (perhaps surprisingly)
s3 and
s4 are also equal when compared
using the equality operator
== that compares the references themselves:
if (s1 == s3) // Condition evaluates to true
However,
s5 and
s6 are not equal (
== ) to
s1 through
s4 ,
because their values are created at runtime and stored separately; therefore,their references are not the same as they are for the literals created at
compile-time.
Always use
equals rather than
== to compare strings, unless you
can explain why the latter is needed!
The assignment operator copies references, not the contents of an object. Given:
int[] a1 = { 1, 2, 3, 4, 5 };int[] a2 = a1;a1[0] = 6;
since
a2 points to the same array, the value of
a2[0] is also changed
to 6. To copy an array, you have to write an explicit loop and copy the elementsone by one.
To copy an object pointed to by a reference, you can create a new object and pass the old object as a parameter to a constructor:
class MyClass {
int x; MyClass(int y) { x = y; }
MyClass(MyClass myclass) { this.x = myclass.x; }}
class Test {
public static void main(String[] args) {
MyClass myclass1 = new MyClass(5); MyClass myclass2 = new MyClass(myclass1);
myclass1.x = 6; System.out.println(myclass1.x); // Prints 6
System.out.println(myclass2.x); // Prints 5 }
}
Alternatively, you can use
clone as described in Java textbooks.
Notification Switch
Would you like to follow the 'Compile and runtime errors in java' conversation and receive update notifications?
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|