Difference between revisions of "Assertions"
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* java -ea -da:net.suhrid.Foo //Enable for all exception of the net.suhrid.Foo class ('''NOTE THE ':'''') | * java -ea -da:net.suhrid.Foo //Enable for all exception of the net.suhrid.Foo class ('''NOTE THE ':'''') | ||
* java -ea:net.suhrid.Bar //Enable only for the net.suhrid.Bar class | * java -ea:net.suhrid.Bar //Enable only for the net.suhrid.Bar class | ||
− | * java -ea -da:net.suhrid.util... //Disable assertions for the net.suhrid.util and all packages below it in the hierarchy. | + | * java -ea -da:net.suhrid.util... //Disable assertions for the net.suhrid.util and all packages below it in the hierarchy. |
+ | * java -ea -da:net.suhrid.util... -ea:net.suhrid.util.Test2 //Disable assertions for the net.suhrid.util and all packages below it in the hierarchy, but enable it for net.suhrid.util.Test2 class. | ||
+ | * java -da -ea:... Test1 //Disable assertions in general but enable assertions in the unnamed package in the current working directory. | ||
* -esa enables assertions in system classes (java.lang). This '''has to be specifically used''', with only -ea systemassertions will still be disabled. | * -esa enables assertions in system classes (java.lang). This '''has to be specifically used''', with only -ea systemassertions will still be disabled. | ||
* -dsa disables system assertions | * -dsa disables system assertions |
Revision as of 09:37, 2 September 2011
Contents
Introduction
- Assertions allow the testing of assumptions about a program.
- Using assertions allow easy detection of bugs during development without writing exception handlers etc.
- We "assert" that something will be true at some point in the code. If it is, code keeps running, if it is false, an AssertionError will be thrown.
Example:
//Without assertions
private static void processAge1(int age) {
if(age > 0) {
System.out.println("Processing age ...");
} else {
System.out.println("Age < 0 !");
}
}
//With assertions
private static void processAge2(int age) {
assert age > 0;
System.out.println("Processing age ...");
}
- Second version of assert takes a second expression which must return a value (primtive/object). The return value can be anything.
- This object's string value is added to the assertion error stack trace.
private static void processAge2(int age) {
assert age > 0 : errorCode();
System.out.println("Processing age ...");
}
private static List<String> errorCode() {
return Arrays.asList("oops");
}
Compiling with assertions
- Assertions were introduced as a keyword in java 1.4.
- Using the source flag - 1.4 onwards trying to use assert as an identifier will result in a compile failure.
- Trying to use -source 1.3 with assert as an identifier will produce warnings and compilation will fail with assert as a keyword.
Running with assertions
- Assertions are disabled by default.
- To enable use the -ea flag.
- To disable use the -da flag. This is useful to disable assertions for specific classes/packages.
Examples:
- java -ea -da:net.suhrid.Foo //Enable for all exception of the net.suhrid.Foo class (NOTE THE ':')
- java -ea:net.suhrid.Bar //Enable only for the net.suhrid.Bar class
- java -ea -da:net.suhrid.util... //Disable assertions for the net.suhrid.util and all packages below it in the hierarchy.
- java -ea -da:net.suhrid.util... -ea:net.suhrid.util.Test2 //Disable assertions for the net.suhrid.util and all packages below it in the hierarchy, but enable it for net.suhrid.util.Test2 class.
- java -da -ea:... Test1 //Disable assertions in general but enable assertions in the unnamed package in the current working directory.
- -esa enables assertions in system classes (java.lang). This has to be specifically used, with only -ea systemassertions will still be disabled.
- -dsa disables system assertions
- For packages the ... notation has to be used.
Correctly using Assertions
- Use assertions to validate arguments to a private method.
- But not to a public one! Use IllegalArgumentException instead. This is because assertions are optional, so it may or may not work.
- Similarly dont use assertions to validate command line arguments.
- Can use assertions in public methods to check for cases that are never supposed to happen. e.g. if we know a switch default must never be reached.
- Don't use asserts that causes values of variables to change - side effects!