Strings
- Can be changed
- Mutable
- String class
S1 = “Hello”
S1 = s1 + “ There” // turns into “Hello There”
- Different ways to declare strings
**char array[] = {‘h’, ‘e’, ‘l’, ‘l’, ‘o’};**
println(array) //prints hello
<span style="text-decoration:underline;">String s = new string(array)</span>; //created a new object of class string naming it s
**String s1= “Hello”;**
**String s2 = “There**
**Literal <span style="text-decoration:underline;">Creating object</span>**Heap Memory
- Inside theres a string pool, or constant pool.
- When we declare that s1 = “Welcome”
- We then search in the heap, inside the heap pool, then we will have a pointer to that string, instead of making a new one
- Same thing when we make another s2 = “Welcome”
- Strings are just null terminated character arrays
- String s3 = new String (“Welcome”)
- Wont make a point to the welcome inside of heap memory’s string pool
- It makes its own object area inside the heap
- When creating string objects you cant use == to check their equality
- You must use a method println(S3.equal(S4)) //will return true or false
- With the normal points to welcome in string pool you can compare (s1 == s2)
- S1 and s2 are string literals, not an object
But why would you make an object string?
- Java is memory efficient by not repeating itself
import java.util.Scanner;
public class string
{
public static void main(String[] args) throws Exception
{
String s1 = "Welcome there";
//extract 5th char there
char ch = s1.charAt(5); //.charAt(index)
System.out.println("ch is "+ch);//output: c
int len = s1.length(); //stores the length of the string
System.out.println("there are "+len+ " characters in the string");
String s2 = "UTA";
String s3 = "UTA";
int res = s1.compareTo(s2); //compareTo method to compare equality of strings, returns a value
System.out.println("Result of comparing S1 and S2 is " + res); //result is 2 so false?
res = s2.compareTo(s3);
System.out.println("Result of comparing S2 and S3 is " + res); // result is 0 which means =
System.out.println("Hello " + s2 + " how are you?");
char A[] = {'h','e','l','l','o', '!'};
String s4 = "";
s4 = s4.copyValueOf(A); //copys value of char array A
System.out.println("copy value of s4 = " + s4);
String s5 = "";
s5 = s5.copyValueOf(A, 3, 3); //copys value of char array A from offset of 3, and counts to end of array only for char array
System.out.println("copy value of s5 = " + s5);
//Done with literals
//now string objs
System.out.println("\n\n\n");
String s6 = new String("Java is fun!");
boolean result;
result = s6.endsWith("fun!"); //endsWith(var or "String") //compares the ending of the strings to each other
System.out.println("Ends with fun ? = " + result);
String s7 ="PC";
String s8 = "pc";
System.out.println("s7 == s8 ? = " + s7.equals(s8)); //checks if s7 and s8 are == by specific characters
System.out.println("s7 == s8 ? = " + s7.equalsIgnoreCase(s8)); //ignores case sensitivity
String s9 = new String("UTA rocks!");
System.out.println("s9 o appears at index = " + s9.indexOf('o')); //returns the first occurance of the letter o
}
}Variable vs objects
- In Java**, no variable can ever hold an object.** A variable can only hold a reference to an object.
- Int a = 5;
- You should think of objects as floating around independently in the computer's memory. In fact, there is a** special portion of memory called the heap where objects live.**
Difference between primitive data types and strings
- Strings are objects!
All primitive data types have veraupper class??
Strings s = “2013”;
println(s); // 2017
println(s + 4); //20134
Parsing strings
- Into usable data that the programmers want
Strings s = “2013”;
- println(s); // 2017
- println(s + 4); //20134
Int a = S.parseInt(); //in theory should convert the string to an integer
- //what a heaven sent method S.parseInt();
- println(a); //2013
- println(a+3); //2016
Wrapper classes
Types of wrapper classes
- Integer
- Long
- Short
- Float
- Double
- Boolean
Int a = 30; //lets create an object of this
Integer b = a; //creates an WRAPPER class object class of integer- This process is known as Auto Boxing
Integer c = 30;
Int d = c;- Going from wrapper class to primitive is Unboxing
double d = Double.parseDouble(“27.3”);//write a java program tha tconverts an integer to its corresponding integer wrapper class and vice versa.
int value = 42;
Integer wrapperValue = Integer.valueOf(value);
int valueAgain = wrapperValue.intValue();
System.out.println("Original int: " + value);
System.out.println("Wrapper to int: " + valueAgain);Tokens
“2014, Java program, 22”
How do we parse this, we cant do parseDatatype();?
- We can parse each of these as a token, but we need to know how they’re separated within our string
- Like for example, our delimiter is a comma
- Other examples are
- “33!JAVVA.C++_WORLD”
- We can have multiple delimiters
- We can tell it extract after finding 1 !, 1 ., and 1 _
- “33!JAVVA.C++_WORLD”
Scanner sc = new Scanner(System.in) //can be any user input or a user file
Lets see how scanner can parse
Scanner sc;
String s = “Hello There. How are you.”;
sc = new Scanner(s, “ ,”);
while( sc.hasNext() ) //hasMoreTokens
{
println( sc.Next() ); //nextTokens
}String tokenizer is similar, but for the while loop is hasmoretokens and nexttoek
Methods of parsing
// Scanner sc;
// String s = ("I love you so much. I want to hug you");
// sc = new Scanner(s);
// while( sc.hasNext() ) //hasMoreTokens
// {
// String token = sc.next();
// System.out.println( token ); //nextTokens
// }
// Scanner sc1;
// String ss = ("I love you so much. I want to hug you");
// sc1 = new Scanner(ss);
// sc1.useDelimiter("[] .]"); //+
// while( sc1.hasNext() ) //hasMoreTokens
// {
// String token = sc1.next();
// System.out.println( token ); //nextTokens
// }
// Scanner sc2;
// String s4 = ("I love you 4 much. I 34 want 22 to hug you");
// sc2 = new Scanner(s4);
// sc2.useDelimiter("[\\+d]"); //+
// while( sc2.hasNext() ) //hasMoreTokens
// {
// String token = sc2.next();
// System.out.println( token ); //nextTokens
// }
String tokens[];
String s4 = ("I love you 4 much. I 34 want 22 to hug you");
tokens = s.split("[\\s\\d]+")
for (int i = 0; i < tokens.length; i++)
{
System.out.println(tokens[i]);
}
System.out.println("number of tokens: " + tokens.length);Preferred way to parse for me
Write a program to split a sentence into words, considering both spaces and commas as delimiters
String str ="Java,Python C++, Swift, Go, Rust, C#, Assembly";
String words[] = str.split("[, \\s]+"); //+ takes spaces or delimeters in a row and sees them as one
for (String word: words)
{
System.out.println(word);
}
output
Java
Python
C++
Swift
Go
Rust
C#
AssemblyArrays
Different ways to declare arrays in java
Can declare like
- Int arr [];
- Int [] arr[];
- Int[] arr[];
In order to allocate memory for these arrays you need to
- Int [] arr;
- Arr = new int[10]
- Cant you do this like int [10] arr; ????
Default initialization of arrays is 0 unlike c where it gives you garbage
Or you can assign it like this
Int arr [/4/] = {1, 2, 3, 4, 5}; //this is obviously 5 values starting from 1 but 4 values starting from 0
- Then we can loop through said array to go through each element in the array. And manipulate it that way.
Array shortcut
For (int a : arr)
println(a);
- Shortcut for looping arrays
2d arrays
Int arr[] = new int[5] {2 , -3 , 10 ,11 ,22};
Int arr[][] name = new int[][] {{2, 3, 5, 7}
,{10, -13},
{1},
{22, -2, 10, 0, 10}};Usually nested for loops to go through each element to do something with it.
I.e.
For ( i = 0; i < name.length; i++_
For ( j = 0; j < name.length; j++)
print(name[i][j]Matrix array shortcut
- To use shortcut do For each:
for( int data[] : name)
for(int num : data)
println(num)