Java基础特性

基本数据类型

整形

浮点类型

char

布尔类型

字符串

String 类型可能是 Java 中应用最频繁的引用类型，但它的性能问题却常常被忽略。高效的使用字符串，可以提升系统的整体性能。

String是不可变的,Java 9 后，String 类的实现改用 byte 数组存储字符串，同时使用 coder 来标识使用了哪种编码

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public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
/**
 * The value is used for character storage.
 *
 * @implNote This field is trusted by the VM, and is a subject to
 * constant folding if String instance is constant. Overwriting this
 * field after construction will cause problems.
 *
 * Additionally, it is marked with {@link Stable} to trust the contents
 * of the array. No other facility in JDK provides this functionality (yet).
 * {@link Stable} is safe here, because value is never null.
 */
@Stable
private final byte[] value;

/**
 * The identifier of the encoding used to encode the bytes in
 * {@code value}. The supported values in this implementation are
 *
 * LATIN1
 * UTF16
 *
 * @implNote This field is trusted by the VM, and is a subject to
 * constant folding if String instance is constant. Overwriting this
 * field after construction will cause problems.
 */
private final byte coder;
}

String不可变的好处？

1. 保证 String 对象安全性，避免 String 被篡改。
2. 缓存hash，保证hash值不会频繁变更。
3. 可以实现字符串常量池。
4. String不可变性天生具备线程安全

.substring(n, m) 截取字符串
String.join("", ...) 拼接

在Java7之前，String Pool 被放在运行时常量池中，它属于永久代。而在Java7，String Pool 被移到堆中。这是因为永久代的空间有限，在大量使用字符串的场景下会导致 OutOfMemoryError 错误。

码点、代码单元

String greeting = "🎁 Hello";

int[] codePoints = greeting.codePoints().toArray();
System.out.println(Arrays.toString(codePoints)); // [127873, 32, 72, 101, 108, 108, 111]
String str = new String(codePoints, 0, codePoints.length);
System.out.println(str); // 🎁 Hello

运算符

浮点运算

strictfp 实现严格的浮点计算

数学函数

数组

Arrays.toString(a); // 打印数组
Arrays.copyOf(); // 拷贝数组