visual basic barcode program a instanceof a instanceof b instanceof b instanceof b instanceof directly! A Foo A B Foo in Java

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a instanceof a instanceof b instanceof b instanceof b instanceof directly! A Foo A B Foo
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// Even though class B doesn t implement Foo
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An object is said to be of a particular interface type (meaning it will pass the instanceof test) if any of the object s superclasses implement the interface.
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In addition, it is legal to test whether a null object (or null itself ) is an instance of a class. This will always result in false, of course. The following code demonstrates this:
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class InstanceTest { public static void main(String [] args) { String a = null; boolean b = null instanceof String; boolean c = a instanceof String; System.out.println(b + " " + c); } }
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When this code is run, we get the following output:
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false false
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So even though variable a was defined as a String, the underlying object is null; therefore, instanceof returns a value of false when compared to the String class.
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Remember that arrays are objects, even if the array is an array of primitives. Look for questions that might look like this:
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int [] nums = new int[3]; if (nums instanceof Object) { } // result is true
An array is always an instance of Object. Any array.
Table 3-2 shows results from several instanceof comparisons. For this table, assume the following:
interface Face { } class Bar implements Face{ } class Foo extends Bar { }
Equality Operators
Equality can be tested with the operators equals and not equals:
== equals (also known as equal to ) != not equals (also known as not equal to )
Equality operators compare two things and return a boolean value. Each individual comparison can involve two numbers (including char), two boolean values, or two
TABLE 3-2
Operands and Results Using instanceof Operator
First Operand (Reference Being Tested) null Foo instance Bar instance Bar instance Foo [] Foo [] Foo[1]
instanceof Operand (Type We re Comparing the Reference Against) Any Class or Interface type Foo, Bar, Face, Object Bar, Face, Object Foo Foo, Bar, Face Object Foo, Bar, Face, Object
Result false true true false false true true
3: Operators and Assignments
object reference variables. You can t compare incompatible types, however. What would it mean to ask if a boolean is equal to a char Or if a Button is equal to a String array (Exactly, nonsense, which is why you can t do it.) There are four different types of things that can be tested:
Numbers Characters Boolean primitives Object reference variables
So what does == actually look at The value in the variable in other words, the bit pattern.
Equality for Primitives
Most programmers are familiar with comparing primitive values. The following code shows some equality tests on primitive variables:
class ComparePrimitives { public static void main(String [] args) { System.out.println("character 'a' == 'a' " + ('a' == 'a')); System.out.println("character 'a' == 'b' " + ('a' == 'b')); System.out.println("5 != 6 " + (5 != 6)); System.out.println("5.0 == 5L " + (5.0 == 5L)); System.out.println("true == false " + (true == false)); } }
This program produces the following output:
%java ComparePrimitives character 'a' == 'a' true character 'a' == 'b' false 5 != 6 true 5.0 == 5L true // Compare a floating point to an int true == false false
As we can see, if a floating-point number is compared with an integer and the values are the same, the == operator returns true as expected.
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Don t mistake = for == in a boolean expression. The following is legal:
1. boolean b = false; 2. if (b = true) { 3. System.out.println( b is true ); 4.} else { 5. System.out.println( b is false ); 6.}
Look carefully! You might be tempted to think the output is b is false, but look at the boolean test in line 2. The boolean variable b is not being compared to true, it s being set to true, so line 3 executes and we get b is true. Keeping in mind that the result of any assignment expression is the value of the variable following the assignment, you can see that in line 3, the result of the expression will be true the value of (b = true). This substitution of = for == works only with boolean variables, since the if test can be done only on boolean expressions. Thus, the following does not compile:
7. int x = 1; 8. if (x = 0) { }
Because x is an integer (and not a boolean), the result of (x = 0) is 0 (the result of the assignment). Integers cannot be used where a boolean value is expected, so the code in line 8 won t work unless changed from an assignment (=) to an equality test (==) as follows:
if (x == 0) { }
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