Assert in Java)

 

We know that C/C ++ has assert ). The assertion feature is also added after Java SE 1.4.

Assertions are used to facilitate program debugging and are not part of a published program. Understanding this is critical.

By default, JVM disables assertions. Therefore, to debug the program using assertions, you must manually enable the assertions function. When running a Java program in command line mode, you can add the parameter-enableassertions or-ea to open the assertions. You can disable assertions through-disableassertions or-da (optional by default ).

Use of assertions:

Assert is defined by the keyword. Generally, assert has two forms.

1. assert <bool expression>; for example, boolean isStudent = false; assert isStudent;

2. assert <bool expression >:< message>; for example, boolean isSafe = false; assert isSafe: "Not Safe at all ";

It is useless. Here are a few simple examples:

 

The first form:

[Java]
01. public class AssertionTest {
02.
03. public static void main (String [] args ){
04.
05. boolean isSafe = false;
06. assert isSafe;
07. System. out. println ("assertion passed! ");
08 .}
09 .}
Public class AssertionTest {

Public static void main (String [] args ){

Boolean isSafe = false;
Assert isSafe;
System. out. println ("assertion passed! ");
}
}

 

If you are running in command line mode, you must specify to enable the assertion function. For example

[Html]
01. java-ea AssertionTest
Java-ea AssertionTest
If it is in IDE, such as Eclipse, you can set it like this: Run as-> Run deployments-> Arguments-> VM arguments: Enter-ea.

 

 

 

Output result:

[Plain]
01. Exception in thread "main" java. lang. AssertionError
02. at AssertionTest. main (AssertionTest. java: 8)
Exception in thread "main" java. lang. AssertionError
At AssertionTest. main (AssertionTest. java: 8)

 

Second form:

[Java]
01. public class AssertionTest {
02.
03. public static void main (String [] args ){
04.
05. boolean isSafe = false;
06. assert isSafe: "Not safe at all ";
07. System. out. println ("assertion passed! ");
08 .}
09 .}
Public class AssertionTest {

Public static void main (String [] args ){

Boolean isSafe = false;
Assert isSafe: "Not safe at all ";
System. out. println ("assertion passed! ");
}
}
Output result:

[Plain]
01. Exception in thread "main" java. lang. AssertionError: Not safe at all
02. at AssertionTest. main (AssertionTest. java: 7)
Exception in thread "main" java. lang. AssertionError: Not safe at all
At AssertionTest. main (AssertionTest. java: 7)

The difference between the second form and the first form is that the latter can specify the error message.

 

Traps:

Assertions are only used to debug programs. Do not write assertions into business logic. For example, consider the following simple example:

[Java]
01. public class AssertionTest {
02.
03. public static void main (String [] args ){
04.
05. assert (args. length> 0 );
06. System. out. println (args [1]);
07 .}
08 .}
Public class AssertionTest {

Public static void main (String [] args ){

Assert (args. length> 0 );
System. out. println (args [1]);
}
}

The assert (args. length> 0) and if (args. length> 0) The meaning is similar, but if the program is released (usually no assertions are enabled), this sentence will be ignored, so the following

[Plain]
01. Exception in thread "main" java. lang. ArrayIndexOutOfBoundsException: 1
02. at AssertionTest. main (AssertionTest. java: 7)
Exception in thread "main" java. lang. ArrayIndexOutOfBoundsException: 1
At AssertionTest. main (AssertionTest. java: 7)
Better alternatives:

JUnit.