GENERICS - Introduction to Java Programming: Comprehensive Version - page 752

a generic type parameter for a class in a static method, field, or initializer. For example, the
following code is illegal:

public class Test<E> {
public static void m(E o1) { // Illegal
}

public static E o1; // Illegal

static {
E o2; // Illegal
}
}

Restriction 4: Exception Classes Cannot Be Generic

A generic class may not extend java.lang.Throwable , so the following class declara-
tion would be illegal:

public class MyException<T> extends Exception {
}

Why? If it were allowed, you would have a catch clause for MyException<T> as follows:

try {
...
}
catch (MyException<T> ex) {
...
}

The JVM has to check the exception thrown from the try clause to see if it matches the
type specified in a catch clause. This is impossible, because the type information is not
present at runtime.

19.18

✓ ✓

What is erasure? Why are Java generics implemented using erasure?

Check

19.19

Point

If your program uses ArrayList<String> and ArrayList<Date> , does the JVM
load both of them?

19.20

Can you create an instance using new E() for a generic type E ? Why?

19.21

Can a method that uses a generic class parameter be static? Why?

19.22

Can you define a custom generic exception class? Why?

19.9 Case Study: Generic Matrix Class
This section presents a case study on designing classes for matrix operations using
generic types.
The addition and multiplication operations for all matrices are similar except that their ele-
ment types differ. Therefore, you can design a superclass that describes the common opera-
tions shared by matrices of all types regardless of their element types, and you can define
subclasses tailored to specific types of matrices. This case study gives implementations for
two types: int and Rational . For the int type, the wrapper class Integer should be used
to wrap an int value into an object, so that the object is passed in the methods for operations.
The class diagram is shown in Figure 19.7. The methods addMatrix and
multiplyMatrix add and multiply two matrices of a generic type E[][] . The static
method printResult displays the matrices, the operator, and their result. The methods
add , multiply , and zero are abstract, because their implementations depend on the
specific type of the array elements. For example, the zero() method returns 0 for the

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