Datasheet

}

public static void main(String args[])

{

CustomHolder2<String> stringHolder = new CustomHolder2<String>();

CustomHolder2<Object> objectHolder = new CustomHolder2<Object>();

String str = new String(“test string”);

String str2;

stringHolder.putItem(str);

objectHolder.putItem(str);

//processHolderObject(stringHolder);

processHolderObject(objectHolder);

processHolderString(stringHolder);

//processHolderString(objectHolder);

processHolderWildcard(stringHolder);

processHolderWildcard(objectHolder);

}

The two lines that are commented will prevent the program from compiling. If both these lines are

uncommented, the compiler issues the following errors:

c:\>javac CustomHolder2.java

CustomHolder2.java:48: processHolderObject(CustomHolder2<java.lang.Object>) in C

ustomHolder2<E> cannot be applied to (CustomHolder2<java.lang.String>)

processHolderObject(stringHolder);

CustomHolder2.java:52: processHolderString(CustomHolder2<java.lang.String>) in C

ustomHolder2<E> cannot be applied to (CustomHolder2<java.lang.Object>)

processHolderString(objectHolder);

2 errors

This reminds you that the type parameter used must match the formal type parameter. However, notice

that neither line that invokes

processHolderWildcard is commented. This is because using the wild-

card allows you to pass in either

stringHolder or objectHolder. You can read the method parameter

type

CustomerHolder2<?> as “a CustomHolder2 of any type” as opposed to “of Object type” or “of

String type.”

A type parameter can be restricted to certain types by what is called a bound. A bound can be applied to

a regular type parameter or a wildcard. Revisit the

Shape example from earlier in the chapter, which

defines a

Shape interface:

import java.util.ArrayList;

import java.util.Iterator;

interface Shape {

void draw();

Part I: Thinking Like a Java Developer

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