String

 String - immutable klasdir. 

package main;

public class Test {
    public static void main(String[] args) {
        String s = null;
        boolean bb = "java".equals(s);
        boolean b = s.equals("java"); // null pointer exception aliriq
    }
}

String x = "hello";
String y = "hello";
System.out.println(x == y);

String a = "hello"; // compile time yaranir
String b = " hello".trim(); // runtime yaranir
System.out.println(a == b);

String l = "Hello World";
String k = "Hello ";
String z = a + "World";
System.out.println(l == z);

    String ll = "Hello World";
    final String kk = "Hello ";
    String zz = kk + "World";
    System.out.println(ll == zz);

String ll = "Hello World";
final String kk = "Hello ";
String tt = "World";
String zz = kk + tt;
System.out.println(ll == zz);

Bes niye bele baw verir. Final variable-lar deyiwilmir ve compiler gorur ki, variable-lar 
deyiwilmir ve menimsedende de compile time baw verir ve ona gorede yuxaridaki misallarin 
bezileri true verir. 

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package main;

public class Main2 {
    public static void main(String[] args) {
        StringBuilder[] a = new StringBuilder[3];
        StringBuilder b = new StringBuilder("Hello");

        for (int i = 0; i < a.length; i++) {
            a[i] = b;
        }

        b.append("Baku");

        for (StringBuilder c : a) {
            System.out.println(c);
        }
    }
}
versus:
package main;

public class Main2 {
    public static void main(String[] args) {
        String[] a = new String[3];
        StringBuilder b = new StringBuilder("Hello");

        for (int i = 0; i < a.length; i++) {
            a[i] = b.toString();
        }

        b.append("Baku");

        for (String c : a) {
            System.out.println(c);
        }
    }
}
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Speed difference between String and StringBuilder and StringBuffer
package main.stirng;

public class StringPerformance {
    public static void main(String[] args) {
//        buildStreamWithPlus("Java", 100_000);
//        buildStringWithStringBuilder("Java", 100_000);
//        buildStringWithStringBuffer("Java", 100_000);
    }

    public static void buildStreamWithPlus(String string, int count) {
        long start = System.currentTimeMillis();
        for (int i = 0; i < count; i++) {
            string = string + i;
        }
        long end = System.currentTimeMillis();
        System.out.println("String. Time to build string is : " + (end - start));
        System.out.println("String length : " + string.length());
    }

    public static void buildStringWithStringBuilder(String string, int count) {
        StringBuilder sb = new StringBuilder(string);
        long start = System.currentTimeMillis();
        for (int i = 0; i < count; i++) {
            sb.append(i);
        }
        long end = System.currentTimeMillis();
        System.out.println("StringBuilder. Time to build string is : " + (end - start));
        System.out.println("String length : " + sb.length());
    }

    public static void buildStringWithStringBuffer(String string, int count) {
        StringBuffer sb = new StringBuffer(string);
        long start = System.currentTimeMillis();
        for (int i = 0; i < count; i++) {
            sb.append(i);
        }
        long end = System.currentTimeMillis();
        System.out.println("StringBuffer. Time to build string is : " + (end - start));
        System.out.println("String length : " + sb.length());
    }
}