- String is Java class backed by char array
- String in immutable (you can’t change the object itself
but can change the reference of the object) since String objects are cache
in String pool. String used to open JDBC and for opening file, so security
can become a thread if class in not immutable
- String is final so no one can override the class
- String overrides equals() and hashCode() methods, two Strings are equal only if they have same characters in same order. Note that equals() method is case sensitive, so if you are not looking for case sensitive checks, you should use equalsIgnoreCase() method.
- We can create string in 2 ways
·
By using new : A new
object is created
String B2=
“Hi”; --O1
String
A1=new String(“Hi”); -- O2
·
By using String literal
“”- checks for similar String literal and if found same reference
String A2= “Hi”; -- Obj1
String B2= “Hi”; same object
String A1=new String(“Hi”); -- Obj2
String B1=new String(“Hi”); -- Obj2
- Immutable: When we modify a string a new object is
created along with the existing one and the reference to the modified is
changed. A,BàObj1 ..after modification Aà Obj1 Bà Obj2
String
A="Hi";
String B ="Hi";
if(A.equals(B)){ -- True
System.out.println("A=B");
}
B=B.toUpperCase();
if(A.equals(B)){ -- False
System.out.println("2 A=B");
}else{
System.out.println("2 A!=B");}
- Since immutable it is thread-safe
- Allows hashcode to be stored in cache so it is fast
- String maintained in String pool more specifically in
PermGen Space
- .equals and == à == refers to references
String personalLoan = new String("loans");
String homeLoan = new String("loans");
//since two strings are different
object result should be false
boolean result = personalLoan == homeLoan; -- False
//since strings contains same conten , equals() should
return true
result = personalLoan.equals(homeLoan);
homeLoan = personalLoan;
//since both
homeLoan and personalLoand reference variable are pointing to same object
//"=="
should return true
result = (personalLoan == homeLoan);
- Important methods indexOf(); lastIndexOf();
matches(String regex)
- Substring
method: can create memory leaks.
Backed by array of char. if beginIndex is equal to
length in substring or beginindex = endindex null string will be returned.
How substring works: it internally calls String (int offset, int count, char value [])constructor to create new String object. In this char value[] is the original string. So if the original string is very long the new string will be equally long. Also, this will stop original string from being garbage collected, in case it does not have live references. Thus creating memory leaks where memory is retained even though it is not required.
Solution:
Simple solution is to trim the string, and keep size of character array
according to length of substring. Or to use intern() method on substring since
it is an original string it will occupy only the required space. Intern() has
to be called on substring and not original string
String str="hello hi
how are you";
String subStr=new String(str.substring(0,3));
Initializes a newly
created String object so that it represents the same sequence of
characters as the argument; in other words, the newly created string is a copy
of the argument string.
subStr.intern();
- + can be used to concat strings. Internally it uses
StringBuffer or StringBuilder which are mutable thus are temporary.
- trim(): trims the leading and trailing spaces in a
string. If removes white spaces then a new string is returned else same
string is returned. Inside trim(), if no trailing and leading whitespaces then
this else substring method is called which in turn returns a new string.
String str1=new String(" hello hi how are you");
String str2=new String("hello");
String trim1=str1.trim();
String trim2=str2.trim();
if(trim1.equals(str1)){
System.out.println("true");
}else{
System.out.println("false"); --False
}
if(trim2.equals(str2)){
System.out.println("true"); --True
}else{
System.out.println("false");
- split() can be used to split a string based on delimiter.
String
str1=new String(" hello
hi how are you");
String[] splitString=str1.split(" ");
String[] splitString=str.split(“\\|”);
- don’t store sensitive information in String since
String is immutable and is stored in string pool(memory), it stays on heap
for a long time and exposes risk of being seen by anyone who has access to
Java memory like reading from dump. Instead char[] should be used to store
password or sensitive information.
- Encoding is important in java specially if u use
languages like German or French.
String
Builder and String Buffer
1.
If string is created
with String literal, it remains in String Pool. Since String is immutable, it
remains in string pool and any method on string creates new String.
2.
StringBuffer is mutable
i.e. you can modify a string buffer without creating a new object.
3.
String Builder is copy
of String Buffer except, String Buffer methods are synchronized and hence is
thread-safe which make is slow at the same time.
4.
Use String if you
require immutability, use
StringBuffer in java if you need mutable + thread-safety and use StringBuilder in Java if you require mutable +
without thread-safety.
