java error countermeasures

This time I will write about the error that appears in java.

Compile error

Here are some compilation errors that are more common to beginners and how to deal with them.

• Missing semicolon ";"
• Insufficient double quotation mark "" "
• Double-byte white space
• File name and class name are different
• Closing bracket "}"

And so on.

Insufficient semicolon ";"

The Java description rule is to add a semicolon ";" to each instruction.

semicolon";Is insufficient

public class HelloWorld {
	public static void main(String[] args) {
		System.out.println("Hello World!")  //← Semicolon ("";”) Is not enough
	}
}

Insufficient double quotation mark "" "

If you don't have enough double quotes "" "at the end of the string, you'll get this error:

Insufficient double quotation mark "" "

public class HelloWorld {
	public static void main(String[] args) {
		System.out.println("Hello World!);  ← World!After the""There is no
	}
}

Double-byte white space

This is caused by putting "full-width blank" characters in the place where "half-width blank" characters should be put.

Half-width white space

public □ class HelloWorld {		   //← Double-byte space after public (□)
	public static void main(String[] args) {
		System.out.println("Hello World!")  
	}
}

File name and class name are different

To give an example Whereas the class name described in the source file is "Hello World" It occurs because the file name is "Hello.java".

In Java, write one class in one file The file name is "class name.java".

Closing bracket "}"

Dozens of lines of programs lack parentheses for paragraphs like if () {} and for () {} It's very difficult to understand.

Closing bracket "}」

	public static void main(String[] args) {
		System.out.println("Hello World!")  
	// ← 「}There is no
}

Syntax error

It gets a little difficult, and I will introduce the error contents that appear in arrays and if syntax.

Click here for the main contents

• java.lang.NullPointerException
• java.lang.ArrayIndexOutOfBoundsException
• java.lang.NumberFormatException
• java.lang.ClassCastException
• java.lang.ArithmeticException
• java.lang.NoClassDefFoundError
• java.lang.OutOfMemoryError

java.lang.NullPointerException This error occurs when trying to access a null object. This error is relatively common during programming and is called "nullpo" by engineers.

Countermeasures

• Check if the instance is created and the value is set correctly.
• Check the timing when the object becomes null by debugging.
• Programmatically check if the object is null and avoid errors as much as possible.

java.lang.ArrayIndexOutOfBoundsException An error that occurs when you specify an element that does not exist as an array index I also introduced the error example earlier. Where there are only 1-9 indexes as array indexes Occurs when trying to set a value for an index of 10.

Countermeasures

• Check if you can specify the index of the correct array.
• If the array is set by iterative processing, check if there is a problem with the number of repetitions.

java.lang.NumberFormatException This error tried to convert (cast) a string value to a number Occurs when an incorrect numerical value is set in the character string.

Countermeasures

• Check if non-numeric characters such as alphabets and symbols are set in the character string.
• Check if the string is set to blank or null.
• A numerical value is set in the character string, but check whether the range of values that can be handled by the numeric type to be converted is exceeded.

java.lang.ClassCastException This error occurs when trying to cast a class to a class that cannot be converted.

Countermeasures

• Check if the class of the value set in the array etc. matches the type you are trying to cast when fetching the value.
• Check if you are trying to cast a superclass to a subclass in two inherited classes.

java.lang.ArithmeticException This error occurs when an illegal arithmetic process is performed in the calculation of a numerical value. For example, this error occurs when a mathematically impossible "division by zero (10 ÷ 0, etc.)" occurs.

Countermeasures

• Check if division by zero is done.
• If you are calculating with a numeric variable, check if the variable to be divided is set to zero.

java.lang.NoClassDefFoundError When calling a method or trying to create an instance This is an error that Java cannot read the class definition.

Countermeasures

• Check the environment variable CLASSPATH as it may be incorrect.
• Check if the file name or package name is correct.
• Check if the external file (jar) required for the program is loaded.

java.lang.OutOfMemoryError Java uses two memory areas at runtime, the stack area and the heap area. This error occurs when the memory size of the heap area required to execute the program is insufficient.

A heap area is a memory area to which an instance of an object is allocated, such as class and method definitions.

Countermeasures

• Check if there is an infinite loop in the program.
• Check if there is a part that is trying to process a large amount of data at once.
• Check if you are using a lot of methods and variables that have a wide scope (public range) unnecessarily.