java备忘录 Java 备忘单

int num = 5;
float floatNum = 5.99f;
char letter = "D";
boolean bool = true;
String site = "quickref.me";

String first = "John";
String last = "Doe";
String name = first + " " + last;
System.out.println(name);

请参阅：字符串

String word = "QuickRef";
for (char c: word.toCharArray()) {
  System.out.print(c + "-");
}
// Outputs: Q-u-i-c-k-R-e-f-

请参阅：循环

char[] chars = new char[10];
chars[0] = "a"
chars[1] = "b"

String[] letters = {"A", "B", "C"};
int[] mylist = {100, 200};
boolean[] answers = {true, false};

请参阅：数组

int a = 1;
int b = 2;
System.out.println(a + " " + b); // 1 2

int temp = a;
a = b;
b = temp;
System.out.println(a + " " + b); // 2 1

// Widening
// byte<short<int<long<float<double
int i = 10;
long l = i;               // 10

// Narrowing 
double d = 10.02;
long l = (long)d;         // 10

String.valueOf(10);       // "10"
Integer.parseInt("10");   // 10
Double.parseDouble("10"); // 10.0

int j = 10;

if (j == 10) {
  System.out.println("I get printed");
} else if (j > 10) {
  System.out.println("I don"t");
} else {
  System.out.println("I also don"t");
}

请参阅：条件

Scanner in = new Scanner(System.in);
String str = in.nextLine();
System.out.println(str);

int num = in.nextInt();
System.out.println(num);

String str1 = "value"; 
String str2 = new String("value");
String str3 = String.valueOf(123);

String s = 3 + "str" + 3;     // 3str3
String s = 3 + 3 + "str";     // 6str
String s = "3" + 3 + "str";   // 33str
String s = "3" + "3" + "23";  // 3323
String s = "" + 3 + 3 + "23"; // 3323
String s = 3 + 3 + 23;        // 29

String s1 = new String("QuickRef"); 
String s2 = new String("QuickRef"); 

s1 == s2          // false
s1.equals(s2)     // true

"AB".equalsIgnoreCase("ab")  // true

String str = "Abcd";

str.toUpperCase();     // ABCD
str.toLowerCase();     // abcd
str.concat("#");       // Abcd#
str.replace("b", "-"); // A-cd

"  abc ".trim();       // abc
"ab".toCharArray();    // {"a", "b"}

String str = "abcd";

str.charAt(2);       // c
str.indexOf("a")     // 0
str.indexOf("z")     // -1
str.length();        // 4
str.toString();      // abcd
str.substring(2);    // cd
str.substring(2,3);  // c
str.contains("c");   // true
str.endsWith("d");   // true
str.startsWith("a"); // true
str.isEmpty();       // false

String str = "hello";
str.concat("world");

// Outputs: hello
System.out.println(str);

String str = "hello";
String concat = str.concat("world");

// Outputs: helloworld
System.out.println(concat);

一旦创建不能修改，任何修改都会创建一个新的String

int[] a1;
int[] a2 = {1, 2, 3};
int[] a3 = new int[]{1, 2, 3};

int[] a4 = new int[3];
a4[0] = 1;
a4[2] = 2;
a4[3] = 3;

int[] a = {1, 2, 3};
System.out.println(a[0]); // 1

a[0] = 9;
System.out.println(a[0]); // 9

System.out.println(a.length); // 3

int[] arr = {1, 2, 3};
for (int i=0; i < arr.length; i++) {
    arr[i] = arr[i] * 2;
    System.out.print(arr[i] + " ");
}
// Outputs: 2 4 6

String[] arr = {"a", "b", "c"};
for (int a: arr) {
    System.out.print(a + " ");
}
// Outputs: a b c 

int[][] matrix = { {1, 2, 3}, {4, 5} };

int x = matrix[1][0];  // 4
// [[1, 2, 3], [4, 5]]
Arrays.deepToString(matrix)

for (int i = 0; i < a.length; ++i) {
  for(int j = 0; j < a[i].length; ++j) {
    System.out.println(a[i][j]);
  }
}
// Outputs: 1 2 3 4 5 6 7 

char[] chars = {"b", "a", "c"};
Arrays.sort(chars);

// [a, b, c]
Arrays.toString(chars);