Java官方筆記3Java語言基礎

變量

Instance Variables (Non-Static Fields) 實例變量（非靜態變量）

一個類可以創造多個實例，實例中的變量叫做實例變量，相互獨立。

Class Variables (Static Fields) 類變量（靜態變量）

對比來看，類變量就是類的變量，一個類只能有一份，不能複製，使用static關鍵字來定義類變量。

代碼：

public class MyVar {
    static int classVar = 1;  // 類變量（靜態變量）
    int instanceVar = 2;  // 實例變量
}

對比Python來看就很清楚：

class MyVar:
    class_var = 1

    def __int__(self):
        instance_var = 2

Local Variables 局部變量

局部變量是由代碼位置決定的，前面的實例變量和類變量，是放到Field位置，而局部變量是放到Method裏面。也就是說，局部變量是放在方法的花括號裏面的變量，並且只能在方法內部訪問，不能被其他地方訪問，這就是所謂的局部。

public class MyVar {
    static int classVar = 1;  // 類變量（靜態變量）
    int instanceVar = 2;  // 實例變量

    void method() {
        int localVar = 3;  // 局部變量
    }
}

Parameters 參數

參數就是方法名後面小括號裏面的變量。

public class MyVar {
    static int classVar = 1;  // 類變量（靜態變量）
    int instanceVar = 2;  // 實例變量

    void method(int parameter) {  // 參數
        int localVar = 3;  // 局部變量
    }
}

變量命名規則

駝峯命名法，可參考阿里規範： https://github.com/alibaba/p3c

基本數據類型

Java有8個基本數據類型：byte、short、int、long、float、double、boolean、char。

對比Python的6個基本數據類型：Number（數字）、String（字符串）、List（列表）、Tuple（元組）、Set（集合）、Dictionary（字典），你發現了什麼嗎？

Java的String嚴格來說不算基本數據類型，因爲它其實是一個類，java.lang.String。

默認值

只聲明不賦值的字段（定義在類級別的field），編譯器會給它賦予默認值：

• byte：0

• short：0

• int：0

• long：0L

• float：0.0f

• double：0.0d

• boolean：false

• char：\u0000

String或任何對象，會賦值爲null。

而對於局部變量（定義在方法裏面的variable），編譯器不會自動賦值，只聲明不賦值，會報編譯錯誤。

類的field會賦默認值。方法的局部variable不會。

字面量Literals

基本數據類型的初始化可以不使用new，而直接使用字面量：

byte b = 100;
short s = 10000;
int i = 100000;
int hexadecimalValue = 0x1a;  // 十六進制
int binaryValue = 0b11010;  // 二進制
long x = 10000000L;
float f1  = 123.4f;
double d1 = 123.4;
double d2 = 1.234e2;  // 科學計數法
boolean result = true;
char capitalC = 'C';

String也不需要new就能初始化：

String s = "this is a string";

另外還有一個字面量.class，比如String.class，用來表示自己的類型。

數組

An array is a container object that holds a fixed number of values of a single type.

注意這裏說的是container object，數組不是基本數據類型，而是容器對象。這一點也可以從數組初始化來論證，數組初始化是需要new的：

int[] anArray;
anArray = new int[10];

代碼示例：

class ArrayDemo {
    public static void main(String[] args) {
        // declares an array of integers
        int[] anArray;

        // allocates memory for 10 integers
        anArray = new int[10];

        // initialize first element
        anArray[0] = 100;
        // initialize second element
        anArray[1] = 200;
        // and so forth
        anArray[2] = 300;
        anArray[3] = 400;
        anArray[4] = 500;
        anArray[5] = 600;
        anArray[6] = 700;
        anArray[7] = 800;
        anArray[8] = 900;
        anArray[9] = 1000;

        System.out.println("Element at index 0: "
                           + anArray[0]);
        System.out.println("Element at index 1: "
                           + anArray[1]);
        System.out.println("Element at index 2: "
                           + anArray[2]);
        System.out.println("Element at index 3: "
                           + anArray[3]);
        System.out.println("Element at index 4: "
                           + anArray[4]);
        System.out.println("Element at index 5: "
                           + anArray[5]);
        System.out.println("Element at index 6: "
                           + anArray[6]);
        System.out.println("Element at index 7: "
                           + anArray[7]);
        System.out.println("Element at index 8: "
                           + anArray[8]);
        System.out.println("Element at index 9: "
                           + anArray[9]);
    }
}

多維數組：

class MultiDimArrayDemo {
    public static void main(String[] args) {
        String[][] names = {
            {"Mr. ", "Mrs. ", "Ms. "},
            {"Smith", "Jones"}
        };
        // Mr. Smith
        System.out.println(names[0][0] + names[1][0]);
        // Ms. Jones
        System.out.println(names[0][2] + names[1][1]);
    }
}

使用System.arraycopy()複製數組：

class ArrayCopyDemo {
    public static void main(String[] args) {
        String[] copyFrom = {
            "Affogato", "Americano", "Cappuccino", "Corretto", "Cortado",
            "Doppio", "Espresso", "Frappucino", "Freddo", "Lungo", "Macchiato",
            "Marocchino", "Ristretto" };

        String[] copyTo = new String[7];
        System.arraycopy(copyFrom, 2, copyTo, 0, 7);
        for (String coffee : copyTo) {
            System.out.print(coffee + " ");
        }
    }
}

使用java.util.Arrays.copyOfRange複製數組：

class ArrayCopyOfDemo {
    public static void main(String[] args) {
        String[] copyFrom = {
            "Affogato", "Americano", "Cappuccino", "Corretto", "Cortado",
            "Doppio", "Espresso", "Frappucino", "Freddo", "Lungo", "Macchiato",
            "Marocchino", "Ristretto" };

        String[] copyTo = java.util.Arrays.copyOfRange(copyFrom, 2, 9);
        for (String coffee : copyTo) {
            System.out.print(coffee + " ");
        }
    }
}

操作符

賦值：

int cadence = 0;
int speed = 0;
int gear = 1;

數學運算：

class ArithmeticDemo {

    public static void main (String[] args) {

        int result = 1 + 2;
        // result is now 3
        System.out.println("1 + 2 = " + result);
        int original_result = result;

        result = result - 1;
        // result is now 2
        System.out.println(original_result + " - 1 = " + result);
        original_result = result;

        result = result * 2;
        // result is now 4
        System.out.println(original_result + " * 2 = " + result);
        original_result = result;

        result = result / 2;
        // result is now 2
        System.out.println(original_result + " / 2 = " + result);
        original_result = result;

        result = result + 8;
        // result is now 10
        System.out.println(original_result + " + 8 = " + result);
        original_result = result;

        result = result % 7;
        // result is now 3
        System.out.println(original_result + " % 7 = " + result);
    }
}

x += 1;等同於x = x + 1;

一元運算符：

class UnaryDemo {

    public static void main(String[] args) {

        int result = +1;
        // result is now 1
        System.out.println(result);

        result--;
        // result is now 0
        System.out.println(result);

        result++;
        // result is now 1
        System.out.println(result);

        result = -result;
        // result is now -1
        System.out.println(result);

        boolean success = false;
        // false
        System.out.println(success);
        // true
        System.out.println(!success);
    }
}

++i和i++都會加1，區別是++i的結果是遞增後的值，i++的結果是原來的值：

class PrePostDemo {
    public static void main(String[] args){
        int i = 3;
        i++;
        // prints 4
        System.out.println(i);
        ++i;               
        // prints 5
        System.out.println(i);
        // prints 6
        System.out.println(++i);
        // prints 6
        System.out.println(i++);
        // prints 7
        System.out.println(i);
    }
}

關係運算符：

class ComparisonDemo {

    public static void main(String[] args){
        int value1 = 1;
        int value2 = 2;
        if(value1 == value2)
            System.out.println("value1 == value2");
        if(value1 != value2)
            System.out.println("value1 != value2");
        if(value1 > value2)
            System.out.println("value1 > value2");
        if(value1 < value2)
            System.out.println("value1 < value2");
        if(value1 <= value2)
            System.out.println("value1 <= value2");
    }
}

條件運算符：

class ConditionalDemo1 {

    public static void main(String[] args){
        int value1 = 1;
        int value2 = 2;
        if ((value1 == 1) && (value2 == 2))
            System.out.println("value1 is 1 AND value2 is 2");
        if ((value1 == 1) || (value2 == 1))
            System.out.println("value1 is 1 OR value2 is 1");
    }
}

?:等同於if-then-else

class ConditionalDemo2 {

    public static void main(String[] args){
        int value1 = 1;
        int value2 = 2;
        int result;
        boolean someCondition = true;
        result = someCondition ? value1 : value2;

        System.out.println(result);
    }
}

Instanceof判斷：an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface

class InstanceofDemo {
    public static void main(String[] args) {

        Parent obj1 = new Parent();
        Parent obj2 = new Child();

        System.out.println("obj1 instanceof Parent: "
            + (obj1 instanceof Parent));
        System.out.println("obj1 instanceof Child: "
            + (obj1 instanceof Child));
        System.out.println("obj1 instanceof MyInterface: "
            + (obj1 instanceof MyInterface));
        System.out.println("obj2 instanceof Parent: "
            + (obj2 instanceof Parent));
        System.out.println("obj2 instanceof Child: "
            + (obj2 instanceof Child));
        System.out.println("obj2 instanceof MyInterface: "
            + (obj2 instanceof MyInterface));
    }
}

class Parent {}
class Child extends Parent implements MyInterface {}
interface MyInterface {}

位運算符：

class BitDemo {
    public static void main(String[] args) {
        int bitmask = 0x000F;
        int val = 0x2222;
        // prints "2"
        System.out.println(val & bitmask);
    }
}

語句

表達式

int cadence = 0;
anArray[0] = 100;
System.out.println("Element 1 at index 0: " + anArray[0]);

int result = 1 + 2; // result is now 3
if (value1 == value2)
    System.out.println("value1 == value2");

語句

// assignment statement
aValue = 8933.234;

// increment statement
aValue++;

// method invocation statement
System.out.println("Hello World!");

// object creation statement
Bicycle myBike = new Bicycle();

塊

class BlockDemo {
     public static void main(String[] args) {
          boolean condition = true;
          if (condition) { // begin block 1
               System.out.println("Condition is true.");
          } // end block one
          else { // begin block 2
               System.out.println("Condition is false.");
          } // end block 2
     }
}

控制語句

if-then

void applyBrakes() {
    // the "if" clause: bicycle must be moving
    if (isMoving){
        // the "then" clause: decrease current speed
        currentSpeed--;
    }
}

if-then-else

class IfElseDemo {
    public static void main(String[] args) {

        int testscore = 76;
        char grade;

        if (testscore >= 90) {
            grade = 'A';
        } else if (testscore >= 80) {
            grade = 'B';
        } else if (testscore >= 70) {
            grade = 'C';
        } else if (testscore >= 60) {
            grade = 'D';
        } else {
            grade = 'F';
        }
        System.out.println("Grade = " + grade);
    }
}

while

class WhileDemo {
    public static void main(String[] args){
        int count = 1;
        while (count < 11) {
            System.out.println("Count is: " + count);
            count++;
        }
    }
}

do-while

class DoWhileDemo {
    public static void main(String[] args){
        int count = 1;
        do {
            System.out.println("Count is: " + count);
            count++;
        } while (count < 11);
    }
}

for

class ForDemo {
    public static void main(String[] args){
         for(int i = 1; i < 11; i++){
              System.out.println("Count is: " + i);
         }
    }
}

enhanced for

class EnhancedForDemo {
    public static void main(String[] args){
         int[] numbers =
             {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
         for (int item : numbers) {
             System.out.println("Count is: " + item);
         }
    }
}

break

class BreakDemo {
    public static void main(String[] args) {

        int[] arrayOfInts =
            { 32, 87, 3, 589,
              12, 1076, 2000,
              8, 622, 127 };
        int searchfor = 12;

        int i;
        boolean foundIt = false;

        for (i = 0; i < arrayOfInts.length; i++) {
            if (arrayOfInts[i] == searchfor) {
                foundIt = true;
                break;
            }
        }

        if (foundIt) {
            System.out.println("Found " + searchfor + " at index " + i);
        } else {
            System.out.println(searchfor + " not in the array");
        }
    }
}

continue

class ContinueDemo {
    public static void main(String[] args) {

        String searchMe = "peter piper picked a " + "peck of pickled peppers";
        int max = searchMe.length();
        int numPs = 0;

        for (int i = 0; i < max; i++) {
            // interested only in p's
            if (searchMe.charAt(i) != 'p')
                continue;

            // process p's
            numPs++;
        }
        System.out.println("Found " + numPs + " p's in the string.");
    }
}

return

return ++count;  // 返回value
return;  // 返回void

Switch語句

int quarter = ...; // any value

String quarterLabel = null;
switch (quarter) {
    case 0: quarterLabel = "Q1 - Winter"; 
            break;
    case 1: quarterLabel = "Q2 - Spring"; 
            break;
    case 2: quarterLabel = "Q3 - Summer"; 
            break;
    case 3: quarterLabel = "Q3 - Summer"; 
            break;
    default: quarterLabel = "Unknown quarter";
};

selector的類型只能是：

• byte short char int 基本數據類型

• Byte Short Character Integer 包裝類型

• 枚舉類型

• String類型

不能是：boolean long float double

不寫break會全部case執行一遍：

int month = 8;
List<String> futureMonths = new ArrayList<>();

switch (month) {
    case 1:  futureMonths.add("January");
    case 2:  futureMonths.add("February");
    case 3:  futureMonths.add("March");
    case 4:  futureMonths.add("April");
    case 5:  futureMonths.add("May");
    case 6:  futureMonths.add("June");
    case 7:  futureMonths.add("July");
    case 8:  futureMonths.add("August");
    case 9:  futureMonths.add("September");
    case 10: futureMonths.add("October");
    case 11: futureMonths.add("November");
    case 12: futureMonths.add("December");
             break;
    default: break;
}

多個case連寫：

int month = 2;
int year = 2021;
int numDays = 0;

switch (month) {
    case 1: case 3: case 5:   // January March May
    case 7: case 8: case 10:  // July August October
    case 12:
        numDays = 31;
        break;
    case 4: case 6:   // April June
    case 9: case 11:  // September November
        numDays = 30;
        break;
    case 2: // February
        if (((year % 4 == 0) && 
             !(year % 100 == 0))
             || (year % 400 == 0))
            numDays = 29;
        else
            numDays = 28;
        break;
    default:
        System.out.println("Invalid month.");
        break;
}

如果selector是null，會報空指針異常：NullPointerException

參考資料：

Java Language Basics https://dev.java/learn/language-basics/