I am implementing a satellite simulation task. What's the logical error that's missing here? [duplicate]

I've been using the == operator in my program to compare all my strings so far. However, I ran into a bug, changed one of them into .equals() instead, and it fixed the bug.

Is == bad? When should it and should it not be used? What's the difference?

Solution 1:

== tests for reference equality (whether they are the same object).

.equals() tests for value equality (whether they are logically "equal").

Objects.equals() checks for null before calling .equals() so you don't have to (available as of JDK7, also available in Guava).

Consequently, if you want to test whether two strings have the same value you will probably want to use Objects.equals().

// These two have the same value
new String("test").equals("test") // --> true 

// ... but they are not the same object
new String("test") == "test" // --> false 

// ... neither are these
new String("test") == new String("test") // --> false 

// ... but these are because literals are interned by 
// the compiler and thus refer to the same object
"test" == "test" // --> true 

// ... string literals are concatenated by the compiler
// and the results are interned.
"test" == "te" + "st" // --> true

// ... but you should really just call Objects.equals()
Objects.equals("test", new String("test")) // --> true
Objects.equals(null, "test") // --> false
Objects.equals(null, null) // --> true

You almost always want to use Objects.equals(). In the rare situation where you know you're dealing with interned strings, you can use ==.

From JLS 3.10.5. String Literals:

Moreover, a string literal always refers to the same instance of class String. This is because string literals - or, more generally, strings that are the values of constant expressions (§15.28) - are "interned" so as to share unique instances, using the method String.intern.

Similar examples can also be found in JLS 3.10.5-1.

Other Methods To Consider

String.equalsIgnoreCase() value equality that ignores case. Beware, however, that this method can have unexpected results in various locale-related cases, see this question.

String.contentEquals() compares the content of the String with the content of any CharSequence (available since Java 1.5). Saves you from having to turn your StringBuffer, etc into a String before doing the equality comparison, but leaves the null checking to you.

Solution 2:

== tests object references, .equals() tests the string values.

Sometimes it looks as if == compares values, because Java does some behind-the-scenes stuff to make sure identical in-line strings are actually the same object.

For example:

String fooString1 = new String("foo");
String fooString2 = new String("foo");

// Evaluates to false
fooString1 == fooString2;

// Evaluates to true
fooString1.equals(fooString2);

// Evaluates to true, because Java uses the same object
"bar" == "bar";

But beware of nulls!

== handles null strings fine, but calling .equals() from a null string will cause an exception:

String nullString1 = null;
String nullString2 = null;

// Evaluates to true
System.out.print(nullString1 == nullString2);

// Throws a NullPointerException
System.out.print(nullString1.equals(nullString2));

So if you know that fooString1 may be null, tell the reader that by writing

System.out.print(fooString1 != null && fooString1.equals("bar"));

The following are shorter, but it’s less obvious that it checks for null:

System.out.print("bar".equals(fooString1));  // "bar" is never null
System.out.print(Objects.equals(fooString1, "bar"));  // Java 7 required

Solution 3:

== compares Object references.

.equals() compares String values.

Sometimes == gives illusions of comparing String values, as in following cases:

String a="Test";
String b="Test";
if(a==b) ===> true

This is because when you create any String literal, the JVM first searches for that literal in the String pool, and if it finds a match, that same reference will be given to the new String. Because of this, we get:

(a==b) ===> true

                       String Pool
     b -----------------> "test" <-----------------a

However, == fails in the following case:

String a="test";
String b=new String("test");
if (a==b) ===> false

In this case for new String("test") the statement new String will be created on the heap, and that reference will be given to b, so b will be given a reference on the heap, not in String pool.

Now a is pointing to a String in the String pool while b is pointing to a String on the heap. Because of that we get:

if(a==b) ===> false.

                String Pool
     "test" <-------------------- a

                   Heap
     "test" <-------------------- b

While .equals() always compares a value of String so it gives true in both cases:

String a="Test";
String b="Test";
if(a.equals(b)) ===> true

String a="test";
String b=new String("test");
if(a.equals(b)) ===> true

So using .equals() is always better.

Solution 4:

The == operator checks to see if the two strings are exactly the same object.

The .equals() method will check if the two strings have the same value.