Java Tutorials - Herong's Tutorial Examples - Version 7.03, by Dr. Herong Yang
Non-Generic Method Example - maxNonGeneric()
This section provides a tutorial example of non-generic method, maxNonGeneric(), that shows type compatibility mistakes are not detected at the compilation time.
To support our analysis provided in the previous section, I wrote the following tutorial example to implement and test the non-generic method:
/* MaxNonGenericMethod.java - Copyright (c) 2014, HerongYang.com, All Rights Reserved. */ class MaxNonGenericMethod { public static Comparable maxNonGeneric( Comparable a, Comparable b) { if (a.compareTo(b)>=0) { return a; } else { return b; } } public static void main(String[] a) { String max = (String) maxNonGeneric( new String("1234"), new String("789")); System.out.println("Maximum: "+max); // Mistake 1: Runtime exception when calling compareTo() String max1 = (String) maxNonGeneric( new String("1234"), new Integer("789")); // Mistake 2: Runtime exception when casting return value Integer max2 = (Integer) maxNonGeneric( new String("1234"), new String("789")); } }
If you compile and run this tutorial example, you will get:
C:\herong>javac MaxNonGenericMethod.java Note: MaxNonGenericMethod.java uses unchecked or unsafe operations. Note: Recompile with -Xlint:unchecked for details. C:\herong>java MaxNonGenericMethod Maximum: 789 Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String at java.lang.String.compareTo(String.java:111) at MaxNonGenericMethod.maxNonGeneric(MaxNonGenericMethod.java:7) at MaxNonGenericMethod.main(MaxNonGenericMethod.java:19)
As you can see from the output, the compiler did not catch the first type compatible mistake in the (String max1 = (String) maxNonGeneric(new String("1234"), new Integer("789"));) call. The resulting bytecode ran into a class cast exception when compareTo() was called.
If comment out line 20 and run the program again:
C:\herong>javac MaxNonGenericMethod.java Note: MaxNonGenericMethod.java uses unchecked or unsafe operations. Note: Recompile with -Xlint:unchecked for details. C:\herong>java MaxNonGenericMethod Maximum: 789 Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer at MaxNonGenericMethod.main(MaxNonGenericMethod.java:23)
As you can see from the output, the compiler did not catch the second type compatible mistake in the (Integer max2 = (Integer) maxNonGeneric(new String("1234"), new String("789"));) call. The resulting bytecode ran into another class cast exception when the return value was assigned to "max2".
Conclusion: Non-generic methods can leave type compatibility mistakes undetected at compilation time. This leads to high likelihood of runtime exceptions.
Last update: 2014.
Table of Contents
Execution Process, Entry Point, Input and Output
Primitive Data Types and Literals
Bits, Bytes, Bitwise and Shift Operations
Managing Bit Strings in Byte Arrays
Reference Data Types and Variables
StringBuffer - The String Buffer Class
System Properties and Runtime Object Methods
Generic Classes and Parameterized Types
►Generic Methods and Type Inference
Comparing Generic Method with Non-Generic Method
►Non-Generic Method Example - maxNonGeneric()
Generic Method Example - maxGeneric()
Generic Methods in java.util.Collections Class
Testing Generic Methods in Collections Class
What Is Type Argument Inference?
Type Argument Inference by Parameter List
Type Argument Inference by Return Value
Generic Methods using Parameterized Types
Parameterized Type as Generic Method Return Type
Lambda Expressions and Method References
Execution Threads and Multi-Threading Java Programs
ThreadGroup Class and "system" ThreadGroup Tree
Synchronization Technique and Synchronized Code Blocks
Deadlock Condition Example Programs