YouTipJava String Contains
## Java String contains() Method
The `contains()` method of the Java `String` class is used to check whether a string contains a specified sequence of characters.
It is a highly efficient, built-in utility for performing substring searches without the need for complex regular expressions or manual index scanning.
---
### Syntax
```java
public boolean contains(CharSequence chars)
```
### Parameters
* **`chars`**: The sequence of characters to search for. This parameter accepts any class implementing the `CharSequence` interface, such as `String`, `StringBuilder`, or `StringBuffer`.
### Return Value
* Returns **`true`** if the string contains the specified sequence of characters.
* Returns **`false`** otherwise.
---
### Code Example
The following example demonstrates how to use the `contains()` method to search for different substrings within a target string:
```java
public class Main {
public static void main(String[] args) {
String myStr = "Runoob";
// Check if the string contains the substring "Run"
System.out.println(myStr.contains("Run")); // Output: true
// Check if the string contains the single character "o"
System.out.println(myStr.contains("o")); // Output: true
// Check if the string contains the character "s"
System.out.println(myStr.contains("s")); // Output: false
}
}
```
**Output:**
```text
true
true
false
```
---
### Key Considerations
When using the `contains()` method, keep the following points in mind:
1. **Case Sensitivity**
The `contains()` method is **case-sensitive**. For example, `"Hello World".contains("world")` will return `false` because of the lowercase "w". If you need a case-insensitive check, convert both strings to lowercase first:
```java
String source = "Hello World";
boolean result = source.toLowerCase().contains("world"); // Returns true
```
2. **NullPointerException**
If you call `contains()` on a `null` string object, or if you pass `null` as the argument, Java will throw a `NullPointerException`. Always ensure your variables are initialized or perform a null check beforehand:
```java
String source = "Hello";
String searchStr = null;
if (source != null && searchStr != null) {
boolean found = source.contains(searchStr);
}
```
3. **Under the Hood**
Internally, the `contains()` method calls `indexOf(CharSequence.toString())`. If `indexOf` returns `0` or greater (indicating the substring was found), `contains()` returns `true`.