Answering the question

First answer should be easiest/simple. Either brute force or first thing that comes to mind

Any optimzation problem

Brute Force
loops
hashing
sorting
space vs time complexity trade offs

Sorting

Merge Sort External Merge Sort Quick Sort

Review

Code snippets

int number = Integer.parseInt("1234");
String str = String.valueOf(1234);

str.charAt(a) // n=0 to length()-1 

char[] characters = sentence.toCharArray();
byte[] byets = sentence.getBytes();

new StringBuilder().append(a).append(b).toString().toString();

String str = str.trim();

String sentence = "Hello World";
String[] words = sentence.split(" ");

"mesquite in your cellar".replace('e', 'o'); // replace by char
str.matches(regex)
str.replaceFirst(regex, repl) 
str.replaceAll(regex, repl)

"unhappy".substring(2) returns "happy"
"hamburger".substring(4, 8) returns "urge"
"to".concat("get").concat("her") returns "together"

StringBuffer is synchronized, StringBuilder is not.
If your text is going to change and use by multiple thread better to use StringBuffer. if your text is going to change but use by single thread then use StringBuilder.

Java is always pass-by-value. The difficult thing can be to understand that Java passes objects as references passed by value.

public void foo(Dog d) {
  d.name.equals("Max"); // true
  d = new Dog("Fifi");
  d.name.equals("Fifi"); // true
}

Dog aDog = new Dog("Max");
foo(aDog);
aDog.name.equals("Max"); // true