Posted on

Difference between a module and a package

https://learnjava.co.in/what-is-the-difference-between-a-module-and-a-package/

Package Module
A package cannot be deployed by itself A module can be deployed by itself
A package groups together related classes A module groups together related packages
Packages are present in Java right from the beginning Modules were added by Java 9
Packages were added to keep related classes together and to allow developers to have a class with the same name in a different packages Modules were added for security reasons and to reduce the size of the JDK
Classes defined within a package are accessible via reflection even if they are private Classes defined within a module are not accessible outside the module via reflection
Packages do not require a package descriptor Modules require a module descriptor which is a file called module-info.java

 

Posted on

String immutable vs StringBuilder mutable


// String immutable vs. StringBuilder mutable
// + operator (new StringBuilder(String.valueOf(str1)).append(str2).toString();
String a = "Hello";
String b = "Hello";
String c = a;
String d = "Hel" + "lo";
String e = "Hello" + "";
String f = a + ""; // String + StringLiteral => new String
String g = "";
String h = a + g; // String + String -> new String
String s = new String("Hello"); // new String
String t = new String("Hello"); // new String
String u = s + new String(""); // String + String -> new String
String v = s + new String(""); // String + String -> new String
String x = s.concat("~"); // concat creates new String
String y = s.concat("~"); // concat creates new String
System.out.println(getReference(a) + " a=" + a + " hashCode=" + a.hashCode());
System.out.println(getReference(b) + " b=" + b + " hashCode=" + b.hashCode());
System.out.println(getReference(c) + " c=" + c + " hashCode=" + c.hashCode());
System.out.println(getReference(d) + " d=" + d + " hashCode=" + d.hashCode());
System.out.println(getReference(e) + " e=" + e + " hashCode=" + e.hashCode());
System.out.println("a==e " + (a == e));
System.out.println(getReference(f) + " f=" + f + " hashCode=" + f.hashCode());
System.out.println("a==f " + (a == f));
System.out.println(getReference(g) + " g=" + g + " hashCode=" + g.hashCode());
System.out.println(getReference(h) + " h=" + h + " hashCode=" + h.hashCode());
System.out.println("a==h " + (a == h));
System.out.println(getReference(s) + " s=" + s + " hashCode=" + s.hashCode());
System.out.println("a==s " + (a == s));
System.out.println(getReference(t) + " t=" + t + " hashCode=" + t.hashCode());
System.out.println("s==t " + (s == t));
System.out.println(getReference(u) + " u=" + u + " hashCode=" + u.hashCode());
System.out.println(getReference(v) + " v=" + v + " hashCode=" + v.hashCode());
System.out.println("u==v " + (u == v));
System.out.println(getReference(x) + " x=" + x + " hashCode=" + x.hashCode());
System.out.println(getReference(y) + " y=" + y + " hashCode=" + y.hashCode());
System.out.println("x==y " + (x == y));

String str1 = "P";
String str2 = "P";
String str3 = str1 + str2;
String str4 = str1.concat(str2);
System.out.println(getReference(str1) + " str1=" + str1 + " hashCode=" + str1.hashCode());
System.out.println(getReference(str2) + " str2=" + str2 + " hashCode=" + str2.hashCode());
System.out.println(getReference(str3) + " str3=" + str3 + " hashCode=" + str3.hashCode());
System.out.println(getReference(str4) + " str4=" + str4 + " hashCode=" + str4.hashCode());
System.out.println("str3==str4 " + (str3 == str4));

 

 
java.lang.String@5a39699c a=Hello hashCode=69609650
java.lang.String@5a39699c b=Hello hashCode=69609650
java.lang.String@5a39699c c=Hello hashCode=69609650
java.lang.String@5a39699c d=Hello hashCode=69609650
java.lang.String@5a39699c e=Hello hashCode=69609650
a==e true
java.lang.String@3cb5cdba f=Hello hashCode=69609650
a==f false
java.lang.String@56cbfb61 g= hashCode=0
java.lang.String@1134affc h=Hello hashCode=69609650
a==h false
java.lang.String@d041cf s=Hello hashCode=69609650
a==s false
java.lang.String@129a8472 t=Hello hashCode=69609650
s==t false
java.lang.String@1b0375b3 u=Hello hashCode=69609650
java.lang.String@2f7c7260 v=Hello hashCode=69609650
u==v false
java.lang.String@2d209079 x=Hello~ hashCode=-2137068020
java.lang.String@6bdf28bb y=Hello~ hashCode=-2137068020
x==y false
java.lang.String@6b71769e str1=P hashCode=80
java.lang.String@6b71769e str2=P hashCode=80
java.lang.String@2752f6e2 str3=PP hashCode=2560
java.lang.String@e580929 str4=PP hashCode=2560
str3==str4 false

Posted on

String + operator vs concat method

The + operator produces a new String by appending the second operand onto the end of the first operand.

The String concat method concatenates the specified string to the end of current string. The first two Strings are NOT changed by the action of concat(). A new String is constructed that contains the character “Red Rose”.

 

How to Concatenate Strings in Java | Developer.com

Strings and Object References in Java: The concat() Method | Saylor Academy