The following are top 10 frequently asked questions about Java Strings.
1. How to compare strings? Use “==” or use equals()?
In brief, “==” tests if references are equal and equals() tests if values are equal. Unless you want to check if two strings are the same object, you should always use equals().
It would be better if you know the concept of string interning.
2. Why is char[] preferred over String for security sensitive information?
Strings are immutable, which means once they are created, they will stay unchanged until Garbage Collector kicks in. With an array, you can explicitly change its elements. In this way, security sensitive information(e.g. password) will not be present anywhere in the system.
3. Can we use string for switch statement?
Yes to version 7. From JDK 7, we can use string as switch condition. Before version 6, we can not use string as switch condition.
// java 7 only!
switch (str.toLowerCase()) {
case "a":
value = 1;
break;
case "b":
value = 2;
break;
} |
4. How to convert string to int?
int n = Integer.parseInt("10"); |
Simple, but so frequently used and sometimes ignored.
5. How to split a string with white space characters?
We can simple do split using regular expression. “\s” stands for white space characters such as ” “, “\t”, “\r”, “\n”.
String[] strArray = aString.split("\\s+"); |
6. Does substring() method create a new string?
The answer is no. The substring() method gives a window to an array of chars which represents the existing String, but do not create a new one. To create a new string, you can do add an empty string like the following:
str.substring(m, n) + "" |
This will create a new string. The above approach sometimes can make your code faster, because Garbage Collector can collect the unused large string and keep only the sub string.
In Oracle JDK 7, substring() creates a new string. Check out the diagram for showing substring() difference between JDK 6 and JDK 7.
7. String vs StringBuilder vs StringBuffer
String vs StringBuilder: StringBuilder is mutable, which means you can modify it after its creation.
StringBuilder vs StringBuffer: StringBuffer is synchronized, which means it is thread-safe but slower than StringBuilder.
8. How to repeat a string?
In Python, we can just multiple a number to repeat a string. In Java, we can use the repeat() method of StringUtils from Apache Commons Lang package.
String str = "abcd"; String repeated = StringUtils.repeat(str,3); //abcdabcdabcd |
9. How to convert string to date?
String str = "Sep 17, 2013";
Date date = new SimpleDateFormat("MMMM d, yy", Locale.ENGLISH).parse(str);
System.out.println(date);
//Tue Sep 17 00:00:00 EDT 2013 |
10. How to count # of occurrences of a character in a string?
Use StringUtils from apache commons lang.
int n = StringUtils.countMatches("11112222", "1");
System.out.println(n); |
One more
Do you know How to detect if a string contains only uppercase letter?
Related Articles:
- Diagram to show Java String’s Immutability
- Java Code – Convert a file to a String
- Why do we need software testing?
- Java Basics
The point no. 6 is no longer true. The Oracle JDK 7+ does not reuse the char[] from the original String, but it create a new one.
Can you share the source code of substring method here? Thanks. In Open JDK 7, it does not create a new one.
From the jdk 1.6.0.18 :
public String substring(int beginIndex, int endIndex) {
if (beginIndex count) {
throw new StringIndexOutOfBoundsException(endIndex);
}
if (beginIndex > endIndex) {
throw new StringIndexOutOfBoundsException(endIndex – beginIndex);
}
return ((beginIndex == 0) && (endIndex == count)) ? this :
new String(offset + beginIndex, endIndex – beginIndex, value);
}
public String substring(int beginIndex, int endIndex) {
if (beginIndex count) {
throw new StringIndexOutOfBoundsException(endIndex);
}
if (beginIndex > endIndex) {
throw new StringIndexOutOfBoundsException(endIndex – beginIndex);
}
return ((beginIndex == 0) && (endIndex == count)) ? this :
new String(offset + beginIndex, endIndex – beginIndex, value);
}
This is the implementation of the method in Oracle JDK 6.
And from JDK7 documentation:
http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#substring(int)
Hi,
check this out: http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7-b147/java/lang/String.java#String.substring%28int%2Cint%29
the substring() method indeed passes the original char[], offset and count, but the constructor in OpenJDK 7 creates a copy of the char[]: http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7-b147/java/lang/String.java#234