Is Java s assertEquals method reliable

Question

I know that    has some issues when comparing two Strings   It seems that String equals   is a better approach   Well  I m doing JUnit testing and my inclination is to use assertEquals str1  str2    Is this a reliable way to assert two Strings contain the same content   I would use assertTrue str1 equals str2    but then you don t get the benefit of seeing what the expected and actual values are on failure   On a related note  does anyone have a link to a page or thread that plainly explains the problems with str1    str2

User · Answer

In a nutshell - you can have two String objects that contain the same characters but are different objects (in different memory locations). The == operator checks to see that two references are pointing to the same object (memory location), but the equals() method checks if the characters are the same.

Usually you are interested in checking if two Strings contain the same characters, not whether they point to the same memory location.

User · Answer

public class StringEqualityTest extends TestCase       public void testEquality   throws Exception           String a    abcde           String b   new String a           assertTrue a equals b            assertFalse a    b           assertEquals a  b

User · Answer

The JUnit assertEquals obj1  obj2  does indeed call obj1 equals obj2    There s also assertSame obj1  obj2  which does obj1    obj2  i e   verifies that obj1 and obj2 are referencing the same instance   which is what you re trying to avoid   So you re fine

User · Answer

assertEquals uses the equals method for comparison  There is a different assert  assertSame  which uses the    operator   To understand why    shouldn t be used with strings you need to understand what    does  it does an identity check  That is  a    b checks to see if a and b refer to the same object  It is built into the language  and its behavior cannot be changed by different classes  The equals method  on the other hand  can be overridden by classes  While its default behavior  in the Object class  is to do an identity check using the    operator  many classes  including String  override it to instead do an  equivalence  check  In the case of String  instead of checking if a and b refer to the same object  a equals b  checks to see if the objects they refer to are both strings that contain exactly the same characters   Analogy time  imagine that each String object is a piece of paper with something written on it  Let s say I have two pieces of paper with  Foo  written on them  and another with  Bar  written on it  If I take the first two pieces of paper and use    to compare them it will return false because it s essentially asking  are these the same piece of paper    It doesn t need to even look at what s written on the paper  The fact that I m giving it two pieces of paper  rather than the same one twice  means it will return false  If I use equals  however  the equals method will read the two pieces of paper and see that they say the same thing   Foo    and so it ll return true   The bit that gets confusing with Strings is that the Java has a concept of  interning  Strings  and this is  effectively  automatically performed on any string literals in your code  This means that if you have two equivalent string literals in your code  even if they re in different classes  they ll actually both refer to the same String object  This makes the    operator return true more often than one might expect

User · Answer

You should always use  equals   when comparing Strings in Java   JUnit calls the  equals   method to determine equality in the method assertEquals Object o1  Object o2    So  you are definitely safe using assertEquals string1  string2    Because Strings are Objects   Here is a link to a great Stackoverflow question regarding some of the differences between    and  equals

User · Answer

Yes  it is used all the time for testing   It is very likely that the testing framework uses  equals   for comparisons such as these   Below is a link explaining the  string equality mistake    Essentially  strings in Java are objects  and when you compare object equality  typically they are compared based on memory address  and not by content   Because of this  two strings won t occupy the same address  even if their content is identical  so they won t match correctly  even though they look the same when printed   http   blog enrii com 2006 03 15 java-string-equality-common-mistake

User · Answer

The    operator checks to see if two Objects are exactly the same Object     http   leepoint net notes-java data strings 12stringcomparison html  String is an Object in java  so it falls into that category of comparison rules

[java] Is Java's assertEquals method reliable?

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