How strings are compared to each other?

When comparing 2 strings, what does actually happen? How do we determine that content inside a String A is equal to String B? Do we look in unicode character table for element to element comparision of both Strings? if yes, then how does equals() know about element in a String like "ABC", how does equal() sperates A from BC to compare it to first element of other string ?

String

2 Answers
2

Strings are basically immutable wrappers around a char, with a lot of useful utility methods.

char

So, to check if two strings are equal, you simply check if they both have char arrays of equal size and with equal content.

char

Using the Arrays class, which has useful methods for working with arrays, a simplified String implementation might look like this:

Arrays

String

public final class String
private char value;

public String(char value)
this.value = value.clone();


@Override
public boolean equals(Object obj)
if (! (obj instanceof String))
return false;
String other = (String) obj;
return Arrays.equals(this.value, other.value);


@Override
public int hashCode()
return Arrays.hashCode(this.value);


public char toCharArray()
return this.value.clone();


public String substring(int beginIndex, int endIndex)
return new String(Arrays.copyOfRange(this.value, beginIndex, endIndex));


// more utility methods




The real String class is more complicated, in different ways depending on the Java version.

String

E.g. in earlier versions, the char could be shared by multiple String objects, so e.g. substring() wouldn't need to copy the array.

char

String

substring()

In later versions, optimizations have changed to store characters in byte (LATIN1) arrays if possible, instead of char (UTF-16) arrays, in order to save space.

byte

char

These various optimizations will of course complicate the internal code of the String utility methods, which might be what is confusing you when you look at the JDK source code.

String

When in Doubt always check the JavaDocs:

Compares this string to the specified object. The result is true if
and only if the argument is not null and is a String object that
represents the same sequence of characters as this object.

When you use equals with Strings this is actually happens

public boolean equals(Object anObject)
if (this == anObject)
return true;

if (anObject instanceof String)
String anotherString = (String)anObject;
int n = value.length;
if (n == anotherString.value.length)
char v1 = value;
char v2 = anotherString.value;
int i = 0;
while (n-- != 0)
if (v1[i] != v2[i])
return false;
i++;

return true;


return false;



So it will return only if you are comparing other String if you have the same sequence because it is comparing Characters.

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