Non-Generic Method Example - maxNonGeneric()

Java Tutorials - Herong's Tutorial Examples

∟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.

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Installing JDK 1.8 on Windows

Execution Process, Entry Point, Input and Output

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