What is complexity of length() function in String class of Java?

Question

Is it O(n) or O(1) (by saving the length in a private variable during string allocation to the object)?

if it is O(n), does it mean that the complexity of following code is O(n^2)?

for(int i=0; i<s.length()-1;i++){
    //some code here!
}

Answer 1

It is O(1) as the length is already known to String instance.

From JDK 1.6 it is visible.

public int length() {
    return count;
}

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Update

It is important to understand why they can cache the value of count and keep using same value for count. The reason lies in a great decision they took when designing String, its Immutability.