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This article mainly explains "what are the basic exercises of the basic grammar class of Java". Interested friends may wish to have a look. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn "what are the basic exercises of Java basic grammar class"?

The use of functional classes

There are some commonly used classes in Java that can be used. Today we are going to learn # about mathematical classes Number class and Math class. One is to box built-in data types into objects, and the other is to perform related operations in mathematics.

Number class

Although there are basic data types in Java syntax, sometimes we need to use data as objects, so Java encapsulates several built-in data types of numbers, such as int, short, long, float, double and byte, into classes, which are Integer, Short, Long, Float, Double, Byte, and they are all subclasses of Number class, inherited from Number class.

And because these classes are modified with the final modifier, they cannot be inherited and rewritten. (as we will talk about later on in inheritance, it probably means that the parent class has, the child class has, and the child class can have what the parent class does not have.)

There are two more wrapper classes for basic data types, one is the Character class and the other is the Booleam class. The Number class is in the java.lang package, and its subclasses are also in java.lang. This wrapper, which is particularly supported by the compiler, is called boxing, so when the built-in data type is used as an object, the compiler boxing the built-in type as a wrapper class.

Similarly, the compiler can unbox an object into a built-in type.

Define the declaration to use the

There is no difference between the method and the built-in or data type: write the type name / class name + space + variable name + assignment operator + value + semicolon. Take Integer as an example:

Public class Te0 {

Public static void main (String [] args) {

Integer a = 10

B = a + 10

System.out.println (b)

}

}

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Running result

twenty

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Other classes are used in the same way

Mtah class

Java's Math contains properties and methods for performing basic mathematical operations, such as elementary index, logarithm, square root and trigonometric functions. Because the Math class is a static class (static), the methods can be called directly, and the same Math class can be used directly without import.

I love math. I love math.

Basic usage

Two important attributes

Since it is a class used for mathematical operations, we have to mention two quantities, one is π\ pi π (pi 3.1415926... ), one is e (natural constant).

These two quantities are also available in the Math class, which are modified by final to represent immutable constants

π\ pi π is Math.PI in the Math of Java

E is Math.E in the Math of Java.

Public class Te1 {

Public static void main (String [] args) {

System.out.println (Math.PI+ "" + Math.E)

}

}

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Output result:

3.141592653589793 2.718281828459045

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Common methods

If you don't say much, we'll go straight to the practical information! Practical information?! Is it really practical information? It is strange that the author can write practical information.

Find the absolute value of abs ()

Math.abs (- 10); / / result: 10

Ceil () rounding up

Math.ceil (5.2); / / result: 6.0directly let individual position + 1

Floor () rounding down

Math.floor (5. 7); / / result: 5. 0 directly let the individual position-1

Maximum value of max ()

Math.max (5,6.2); / / result: 6.2 compares the size of two numbers and returns the larger

The b power of pow () a

Math.pow (2,3); / / result: 8.0 returns a result of 2percent, and the result is of type double.

Random () random number

Math.random (); / / result: a random number of 0-1. Note that there are no parameters in it.

Round () is rounded

Math.ronud 11.1; / / result: 11

Sqrt () to find the square root

Math.sqrt (2); / / result: 1.4142135623730951

Sin () cos () tan () asin () acos () atan () trigonometric function and inverse trigonometric function

After a simple method, you have to practice? Or take a look at the results of the operation? Yeah, here we go.

System.out.println (Math.sin (Math.PI/2)); / / result: 1.0 to find the sine value

System.out.println (Math.cos (Math.PI/2)); / / result: 6.123233995736766E-17 calculates the cosine

System.out.println (Math.tan (Math.PI/2)); / / result: 1.633123935319537E16 calculates the tangent

System.out.println (Math.asin (0.5); / / result: 0.5235987755982989 calculate the inverse function value of sin

System.out.println (Math.acos (0.5)); / / result: 1.0471975511965979 calculate the inverse function of cos

System.out.println (Math.atan (0.5)); / / result: 0.4636476090008061 to calculate the inverse function of tan

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String related classes

A total of these: String, StringBuffe, StringBuilder three kinds of String can not be modified, when modifying the string, you need to use the StringBuffer and StringBuilder classes.

The biggest difference between StringBuilder and StringBuffer is that the method of StringBuilder is not thread-safe (cannot be accessed synchronously).

Because StringBuilder has a speed advantage over StringBuffer, the StringBuilder class is recommended in most cases

String class

Define

Direct definition method

String str = "Hello World!"

Object creation method

String str1 = new String ("Hello World!")

What is the difference between the two methods? First, if the string content is the same, then two variables will point to the same string, which can be understood as a person has more than one name (screen name); if the second content is the same, comparing them with the equals () method will return false, because they are two instantiated objects, which can be understood as: there are two people with the same name in the world, but not one person.

Use

String concatenation

Use + sign stitching

System.out.println ("Hello" + "" + "World" + "!"); / / result: Hello World!

Use the concat method

System.out.println ("Hello" .concat ("World!")); / / result: Hello World!

String formatting

It is through some kind of symbol to format the string output, it can be understood like this: if the string I want to use is variable, for example, when I run, I shout every lap: run x circle. Then we can use string formatting.

Some people will say, when I used it before, didn't I use the + sign?

Yes, you can use the + sign, but if there are multiple quantities, won't + seem a little cumbersome?

So after we have learned, we can use the method of formatting strings, which is not only high, but also easy to use, isn't it beautiful?

System.out.printf ("He%slo Wo%sld!", "l", "r")

/ / the result of the first method is that Hello World% s represents a placeholder and can represent any type of

/ / the same placeholder also has% d, which represents the numeric

System.out.println (String.format ("He%slo Wo%sld!", "l", "r"))

/ / the result of the second method is Hello World!

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String comparison

System.out.println ("a" .equals ("b")); / / result: false

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The string finds the subscript by element

System.out.println ("appdata" .indexOf ("t")); / / the result is: 5 if not found,-1 is returned

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String length

System.out.println ("appdata" .length ())

/ / the result is: 7 returns the length of the string

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StringBuilder class

Unlike String, the object of the StringBuilder class is mutable, which means that we can operate directly on the original object without generating a new object.

Define and use

StringBuilder is a class, so instantiate the object through the new keyword

StringBuilder S1 = new String ("hello world"); / / write the string directly for initialization

StringBuilder S2 = new StringBuilder (5); / / define a length without initialization

System.out.println (S1 + "" + S2)

S1.append ("12345")

S1.append ("666")

System.out.println (S1 + "" + S2)

/ *

The output is as follows:

1234512345666 12345

, /

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Common methods:

String .append (); A parameter adds new content to the original string. The first four steps above are to add 12345 to the empty string.

StringBuilder S1 = new StringBuilder ("hello world")

S1.append (!)

System.out.println (S1)

/ / result: hello world!

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String .insert (); two parameters

StringBuilder S1 = new StringBuilder ("hello world")

S1.insert (10 "!")

System.out.println (S1)

/ / result: hello world!

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The first parameter is the string subscript, and the second is the content, which is added before the subscript

3. The string .delete () has two parameters; both parameters are subscripts, the first is the starting subscript and the second is the terminating subscript, deleting the content between the two subscripts.

StringBuilder S1 = new StringBuilder ("hello world!")

S1.delete (0,4)

System.out.println (S1)

/ / the result is: world!

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StringBuffer class

Like the StringBuilder class, the StringBuffer class is a mutable object.

Define

StringBuffer S1 = new StringBuffer ("123")

/ / instantiate the object to S1 and initialize it to 123

StringBuffer S1 = new StringBuffer ()

/ / instantiate the object to S1 without initialization

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Use

StringBuffer S1 = new StringBuffer ("123")

System.out.println (s1.append ("1")); / / append 1, output: 1231

System.out.println (s1.replace ("1", "2")); / / replace 1 with 2, output: 2232

System.out.println (s1.insert (3, "2")); / / insert 2 after subscript 3, output: 22322

System.out.println (s1.delete (1,2)); / delete values from subscript 1 to subscript 2, output 222,

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That's all, but as I said at the beginning, these are all basic exercises, so that's it.

Scanner class and random class

One is the class that controls the input, and the other is the random number class.

Scanner class

Get the data from the keyboard, which is located under the java.util package. You need to import it first.

1. Upper hand

Import first

Import Java.util.Scanner

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Re-instantiate

Scanner scan = new Scanner (System.in)

/ / instantiate a class named scan for the Scanner class to receive data obtained from the keyboard

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Use

Int a = scan.nextInt ()

/ / declare the integer variable a, and store the data obtained from the keyboard in a. Input must be an integer, otherwise an error will be reported.

Float b = scan.nextFloat ()

/ / declare a single-precision floating-point variable b, and store the data obtained from the keyboard in b. The input must be a single-precision floating-point number, otherwise an error will be reported.

Char c = scan.nextChar ()

/ / declare the character variable c, and store the data obtained from the keyboard in c. The input must be of character type, otherwise an error will be reported.

String d = scan.next ()

/ / declare the string d, and store the data obtained from the keyboard in d. The input must be a string, otherwise an error will be reported.

System.out.println (a + "" + b + "" + c + "" d)

/ / you can call them through variable names as you would with ordinary variables.

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All right, no, that's all I can do. I'm short of knowledge.

Random class

Random number class, simple and practical, really super simple usage

Define

Random rd = new Random ()

/ / the first method: instantiate a Random class object

Random rd2 = new Random (10)

/ / the second method: instantiate an object according to the seed value

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Use

System.out.println (rd.nextInt ()); / / generate a random integer

System.out.println (rd.nextDouble ()); / / generate a 0-1 double precision floating point number

/ / … Among other things, the use of the name of the basic data type in uppercase next+ is similar to the way Scanner receives data

System.out.println (rd.nextInt (5)); / / generate a random integer between 0 and 5

System.out.println (rd.nextDouble () * 5); / / generate a decimal of 0-5

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At this point, I believe you have a deeper understanding of "what are the basic exercises of the basic grammar class of Java?" you might as well do it in practice. Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!

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