Difference between System.Assert and System.AssertEquals

Use System.assert and System.assertEquals together in a negative test case to validate DML exception behavior. The following pattern describes how to test that a record with an invalid field value (for example, a mileage value above the 500-mile daily limit) correctly fails with a validation exception.
Run the test in a System.runAs(u3) context to simulate the specific user context. Inside System.runAs, attempt to insert a Mileage__c record with a Miles__c value of 501 (which exceeds the 500-mile limit). Wrap the insert in a try/catch block that catches DmlException.
In the catch block, use three assertions:

1. System.assert() — Verify that the exception message contains the expected text: 'Insert failed. First exception on row 0; first error: FIELD_CUSTOM_VALIDATION_EXCEPTION, Mileage request exceeds daily limit(500): [Miles__c]'. Pass the actual exception message as the optional second parameter to display it on failure.
2. System.assertEquals(Mileage__c.Miles__c, e.getDmlFields(0)[0]) — Confirm that the DML field that triggered the exception is Miles__c.
3. System.assertEquals('FIELD_CUSTOM_VALIDATION_EXCEPTION', e.getDmlStatusCode(0)) — Confirm that the DML status code is FIELD_CUSTOM_VALIDATION_EXCEPTION.
   
   

static testMethod void runNegativeTestCases(){
	User u3 = [select id from User where alias='tuser'];     
	System.RunAs(u3)
	{
		System.debug('Inserting a record with 501 miles... (negative test case)');

		Mileage__c testMiles3 = new Mileage__c( Miles__c = 501, Date__c = System.today() );
		try{

			insert testMiles3;

		}catch (DmlException e){

			//Assert Error Message 

			System.assert( e.getMessage().contains('Insert failed. First exception on ' +'row 0; first error:FIELD_CUSTOM_VALIDATION_EXCEPTION,'+'Mileage request exceeds daily limit(500): [Miles__c]'),e.getMessage() );

			//Assert field 

			System.assertEquals(Mileage__c.Miles__c, e.getDmlFields(0)[0]);

			//Assert Status Code 

			System.assertEquals('FIELD_CUSTOM_VALIDATION_EXCEPTION' , e.getDmlStatusCode(0) );

		} //catch 

} //RunAs(u3)