Generics Wildcards

Module: Generics and Collections
Topic: Generics Wildcards

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📋 Table of Contents

• Q1: If java.lang. Object is the super type for java.lang. Number and, Number is the
• [Q2: How will you go about deciding which of the following to use?

for (Object val : vals) { System.out.println ("read: " + val); } }

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🔹 Q5: Where will you apply the wild card “ ? super T “?

Answer:

If you were to add the following method to “GenericSingleT ypeScenario.java” The following code will compile.mport java.util.ArrayList ; mport java.util.List; public class GenericSingleT ypeScenarioT est { public static void main (String [] args) { GenericSingleT ypeScenario listOfInts = new GenericSingleT ypeScenario (); listOfInts .add(1); // can add Number or subclasses of Number

List fruits = new ArrayList (); fruits .add("Apple" );

listOfInts .read(fruits ); } public void add(List<? super T> list, T val) { vals.add(val); }

Output: The following code without the wildcard “? super T”mport java.util.ArrayList ; mport java.util.List; public class GenericSingleT ypeScenarioT est { public static void main (String [] args) { GenericSingleT ypeScenario listOfInts = new GenericSingleT ypeScenario (); listOfInts .add(2);

List doubles = new ArrayList (); doubles .add(5.0);

listOfInts .add(doubles, 6);

System.out.println (doubles ); } 5.0, 6]

will throw a compile error as shown below as “ T” is an “Integer” and List cannot be assigned to add(…..) method’ s first argument List But it will be able to assigned to List<? super Integer> as “Number” is a super class of “Integer”. Have you completed this unit? Then mark this unit as completed. Mark as Completed « Previous Unit Next Unit »public void add(List list, T val) { list.add(val); } mport java.util.ArrayList ; mport java.util.List; public class GenericSingleT ypeScenarioT est { public static void main (String [] args) { GenericSingleT ypeScenario listOfInts = new GenericSingleT ypeScenario (); listOfInts .add(2);

List doubles = new ArrayList (); doubles .add(5.0);

listOfInts .add(doubles, 6 // !!!! COMPILE ERROR

System.out.println (doubles ); }

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Source: Extracted from PDF Last Updated: 2026-06-03

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📚 Related Topics

• Collections Overview
• Generics Introduction
• OOP Introduction

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💡 Key Takeaways

Review the questions above and ensure you understand:

• Core concepts and their practical applications
• Real-world scenarios and use cases
• Best practices and common pitfalls

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