Java Basic

字符串操作

字符串比较

boolean result = str1.equals(str2);
boolean result = str1.equalsIgnoreCase(str2);

搜索与检索

int result = str1.indexOf(str2);
// 从字符串的第 index 个数开始查找
int result = str1.indexOf(str2, index);
// 从第 index1 数取到最后
String index = str1.substring(index1);
//  范围 [index1, index2)
String index = str1.substring(index1, index2);

字符串反转

StringBuilder sb = new StringBuilder(string);
String str = sb.reverse().toString();

按单词的字符串反转

String str = "hello world";
Deque<String> stack = new ArrayDeque();
StringTokenizer strTok = new StringTokenizer(str);
while(strTok.hasMoreTokens()){
        stack.push(strTok.nextElement());
}
StringBuilder sb = new StringBuilder();
while(!stack.empty()) {
sb.append(stack.pop());
sb.append(" ");
}

大小写转化

String strUpper = str1.toUpperCase();
String strLower = str1.toLowerCase();

首尾空格移除

String str1 = "     asdfsdf   ";
str1.trim();

空格移除

str1.replace(" ","");

字符串转化为数组

String str = "tim,kerry,timmy,camden";
String[] results = str.split(",");

数据结构

重置数组大小

int[] myArray = new int[10];

int[] tmp = new int[myArray.length + 10];
System.arraycopy(myArray, 0, tmp, 0, myArray.length);
myArray = tmp;

Map 遍历

for (Iterator it = map.entrySet().iterator();it.hasNext();){
            Map.Entry entry = (Map.Entry)it.next();
            Object key = entry.getKey();
            Object value = entry.getValue();
}

映射集合

HashMap<Type> map = new HashMap<>();
map.put(key, value);
// find key or value
map.containsKey(key);
map.containsValue(value);
// 将 hashmap 转化为 array
Object[] objects = hashmap.entrySet().toArray();

数组和列表

int[] nums = {1,4,7,324,0,-4};
Arrays.sort(nums);
// binary search
int index = Arrays.binarySearch(nums, number);
// 列表排序
Collections.sort(List)
// arrayList 转化为 array
Object[] objects = arrayList.toArray();

Set

• TreeSet排序默认是升序，想要实现自定义排序可以通过传进去一个Comparator或者TreeSet的添加对象实现Comparator接口

• 遍历

  • foreach 遍历

  • 迭代器遍历

    Iterator<Integer> iterator =hashSet.iterator();
    while (iterator.hasNext()){
       Integer integer=iterator.next();
    }

  • 需要重写 hashCode() 和 equals() 方法

基本数据结构

比较 Double

Double a = 4.5;
Double b = 4.5;
boolean result = a.equals(b);

二进制、八进制、十六进制转换

int val = 25;
String binaryStr = Integer.toBinaryString(val);
String octalStr = Integer.toOctalString(val);
String hexStr = Integer.toHexString(val);

随机数生成

double rn = Math.random();
// 生产正太分布的数
// mean, var
double value = Math.sqrt(var)*random.nextGaussian()+mean

格式化数字

double value = 2343.8798;
NumberFormat numberFormatter = NumberFormat.getNumberInstance();
String formattedValue = numberFormatter.format(value);
System.out.format("%s%n",formattedValue);