Java Community | Help. Code. Learn.•3mo ago

prepping for an exam, new to java & might need help understanding some stuff

im just gonna put in questions here as I study ill probably not ask a whole ass program explanation unless its fully indecipherable to me ill bold whatever is the tldr version if you dont want extra context

31 Replies

JavaBot•3mo ago

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Piglet | **NEED** tone tags!!OP•3mo ago

firstly, whats a null literal 😭 ik it represents like no value but the text says "null is used to initialize an object of a class ... and may denote non-availability of a character in a String" secondly, ill need to do conversions from/to decimal, binary, hexadecimal and octal numbers but if anyone happens to know a method thats easy for any of that pls lmk

Piglet | **NEED** tone tags!!OP•3mo ago

this is what we've been taught, I just have some trouble w I. & II. cuz im not great at calculations

dan1st•3mo ago

null is a special value to indicate that there is no object. https://stackoverflow.com/a/19654040/10871900 The null literal is the text null that refers to that value

Stack Overflow

What is the difference between null, 0 and nothing?

What is the difference between null, 0 and nothing? I cannot find a question addressing all three of these. For example: If I am getting input from a string and parsing it to int. return Integer.

dan1st•3mo ago

So every reference variable (so variables that are not primitives like boolean, char, short, int, long, float, double can be null meaning "no value associated with it" and you can explicitly set a variable to null using yourVariable = null; or check whether a variable is null using if(yourVariable == null) {}

dan1st•3mo ago

Stack Overflow

What is null in Java?

What is null? Is null an instance of anything? What set does null belong to? How is it represented in the memory?

Piglet | **NEED** tone tags!!OP•3mo ago

okay so like, its ONLY for non-primitive datatypes, but if I wanted to do pretty much the same thing w a primitive data type, smth like int example = 0 or String ex2 = "" works right

dan1st•3mo ago

Integer.parseInt and Integer.toString, both support a radix specifying the base (e.g. 2 for binary, 10 for decimal, 16 for hexadecimal) https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Integer.html#parseInt(java.lang.String,int) https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Integer.html#toString(int,int)

Piglet | **NEED** tone tags!!OP•3mo ago

oh I cant use code

dan1st•3mo ago

The value 0 is a normal int and Strings are not primitive so a String can be null (which is usually not that good of an idea)

Piglet | **NEED** tone tags!!OP•3mo ago

its a written exam, might have to write some programs but other than a simple calculator I cant use anything else okay, thanks

dan1st•3mo ago

Well you can still use Integer.parseInt om your cpde

Piglet | **NEED** tone tags!!OP•3mo ago

I mean the questions for that are gonna be like: "Convert 110101001 to hexadecimal" and id have to work it out myself

dan1st•3mo ago

String hexadecimal = "A5";//a string with a number in hexadecimal
int convertedToInt = Integer.parseInt(hexadecimal, 16);//convert the hex (16) string to an int for further processing
String toBinary = Integer.parseInt(convertedToInt, 2);//convert the int to a binary (2) string
String toDecimal = Integer.parseInt(convertedToInt, 10);//convert the int to a decimal (10) string

String hexadecimal = "A5";//a string with a number in hexadecimal
int convertedToInt = Integer.parseInt(hexadecimal, 16);//convert the hex (16) string to an int for further processing
String toBinary = Integer.parseInt(convertedToInt, 2);//convert the int to a binary (2) string
String toDecimal = Integer.parseInt(convertedToInt, 10);//convert the int to a decimal (10) string

oh you mean without code? There are a few techniques for it

Piglet | **NEED** tone tags!!OP•3mo ago

for program questions theyre like "A Piglatin word is the part of the word after the first vowel, the part before, added with an 'ay'" and I'd have to use like substring and loop commands to get that output yup

dan1st•3mo ago

You can convert a binary number to decimal like this: The leftmost bit is 1, then 2, then 4, then 8, etc - you multiply each bit with the corresponding number and add them up For converting decimal to hex, you divide it by 16 and take the remainder. That's the last digit. You continue using the result of the division and repeat For binary<--> hex conversion specifically, it's easier you just split it in groups of 4

Piglet | **NEED** tone tags!!OP•3mo ago

yeah

dan1st•3mo ago

and then you convert each group to a decimal number and then that decimal number is the hex digit (ofc use ABCDEF for numbers between 10 and 16)

Piglet | **NEED** tone tags!!OP•3mo ago

okay I just wanted to check if anyone knew a quicker method but thats what we've been taught so thats resolved ig whats the (String args[]) part of public static void main (String args[]) for?

dan1st•3mo ago

these are program arguments

Piglet | **NEED** tone tags!!OP•3mo ago

like ik that its the main method class, but ive seen (String []args) and just now (String p)

dan1st•3mo ago

For main arguments specifically: You can start a program using java YourMainClass.java/java your.fully.qualified.Main/java -jar yourjar.jar. You can also add additional text after that. So you'd start it with java YourMainClass.java your arguments here/java your.fully.qualified.Main your arguments here/java -jar yourjar.jar your arguments here. In this case, args woulöd have the Strings your, arguments and here And parameters in general: If you declare a method like this:

public static void someMethod(){
  //...
}

public static void someMethod(){
  //...
}

then you can call it with someMethod(); but if you declare a method like this:

public static void someMethod(String someString){
  //...
}

public static void someMethod(String someString){
  //...
}

then you can call it with someMethod("Hello World"); (or with any other String) and you can use the passed value in the method:

public static void someMethod(String someString){
  System.out.println(someString);
}

public static void someMethod(String someString){
  System.out.println(someString);
}

dan1st•3mo ago

Dev.java: The Destination for Java Developers

Defining Methods - Dev.java

Adding methods to a class definition.

Dev.java: The Destination for Java Developers

Calling Methods and Constructors - Dev.java

How to pass information to a method or a constructor.

Piglet | **NEED** tone tags!!OP•3mo ago

hm im getting some obsidian file linking deja vu OHHH aliases

dan1st•3mo ago

Why? ?

Piglet | **NEED** tone tags!!OP•3mo ago

like if a note is called 'Monkeys' you can add alternative titles or aliases within the note and while linking it in another note both will work as links, like both show up as options and idk that feels kinda similar to this thing, not too much of a similarity since the (String somestring) isnt exactly an alias, but since both someMethod and someMethod("Hello World") works its just enough to remind me of it

Piglet | **NEED** tone tags!!OP•3mo ago

wierd comparison probably but idk anyw im taking a break

dan1st•3mo ago

I know obsidian linking but I don't really see the connection If you pass a primitive parameter, it copies the value but objects are references to the same object and if you pass that, you get a new reference to the same object ig that might be what you mean

Piglet | **NEED** tone tags!!OP•3mo ago

idk my brain just understands shit by trying to figure out if it resembles smth ik already so that can mean the most obscure connections that even I dont really understand

dan1st•3mo ago

That's perfectly fine as long as you don't make too many assumptions thinking it would work similarly

JavaBot•3mo ago

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prepping for an exam, new to java & might need help understanding some stuff