java.lang package common classes

java.lang Classes in the package are automatically imported and usually do not need to be written when used import Statement.

Object class

Object is the root class of the Java class hierarchy and all classes inherit it directly or indirectly.

toString() method

toString() Returns the string representation of the object. The default format is usually defined by the class name,@ and the hexadecimal form of the hash code, but it is not a reliable memory address.

@Override
public String toString() {
    return "Person{name='" + name + "', age=" + age + "}";
}

equals() method

Object.equals() Default and == The effect is the same, comparing whether two references point to the same object. Subclasses can override this and compare object content instead.

rewrite equals() Sometimes it is also rewritten hashCode(), ensure that objects with equal content have the same hash code.

hashCode() method

hashCode() Returns an integer hash value, commonly used in HashMap、HashSet Wait for the hash structure. The same hash code does not mean that objects must be equal, but equals() Equal objects must have the same hash code.

clone() method

clone() Used to copy objects. called directly Object.clone() Class of typically needs to implement Cloneable Interface, otherwise it will throw CloneNotSupportedException。The default clone is a shallow copy, and the reference field may still point to the same object.

finalize() method

finalize() There is no guarantee when it will be executed, nor is it suitable to release resources such as files and network connections. should be used try-with-resources Or explicitly turn off methods to manage resources, do not rely on finalize()。

wrapper class

All eight basic types have corresponding packaging categories.

Wrapper classes encapsulate basic type values into objects for easy use in scenarios such as generic collections that only accept reference types.

Integer Common methods

recommended Integer.valueOf() created or retrieved Integer objects, using Integer.parseInt() Convert a string to int。

Integer value1 = Integer.valueOf(3);
Integer value2 = Integer.valueOf("3");
int number1 = value1.intValue();
int number2 = Integer.parseInt("100");

Integer.valueOf() Cache objects that reuse some commonly used values. The Java specification requires at least caching -128 to 127, a larger cache range may be determined by JVM configuration.

Integer value1 = Integer.valueOf(3);
Integer value2 = Integer.valueOf(3);
System.out.println(value1 == value2); // 通常为 true

Integer value3 = Integer.valueOf(300);
Integer value4 = Integer.valueOf(300);
System.out.println(value3 == value4); // 不应依赖该结果

When comparing the numerical content of packaging categories, use the equals(), do not use == Determine the content of the object.

Automatic boxing and automatic unpacking

JDK 5 begins to support automatic boxing and automatic unpacking.

Integer value = 3; // 自动装箱，相当于 Integer.valueOf(3)
int number = value; // 自动拆箱，相当于 value.intValue()

If the packaging class is referenced as null, will throw NullPointerException。

Integer value = null;
// int number = value; // 运行时抛出 NullPointerException

String class

String Represents an immutable sequence of characters. After a string object is created, its character content cannot be modified; what appears to be modifying a string will actually return a new string.

String text = "abcdefg";
String result = text.replace("bcd", "BCD");

System.out.println(text);   // abcdefg
System.out.println(result); // aBCDefg

String realized CharSequence Interface, therefore requires CharSequence The position of the parameter can be passed in a string.

Internal storage of String

Different JDK versions have different implementations. Java 8’s String mainly used char[] Saving content; newer JDK usage byte[] Use the encoding mark to achieve a compact string. learn and use String APIs should not rely on specific internal array types.

String constant pool

String literals are put into the string constants pool, and the same literals are usually reused on the same object. use new String() A new object is explicitly created.

String s1 = "ab";
String s2 = "ab";
System.out.println(s1 == s2); // true

String s3 = new String("ab");
String s4 = new String("ab");
System.out.println(s3 == s4); // false
System.out.println(s3.equals(s4)); // true

String constant expressions that can be determined during compilation time are collapsed.

String s1 = "ab";
String s2 = "a" + "b";
System.out.println(s1 == s2); // true

Splices that contain variables usually produce result objects during run time and should not be used == Compare string content.

String Common methods

Get length and characters

String text = "abcdefg";
System.out.println(text.length());
System.out.println(text.charAt(2));

compare strings

equals() Compare whether the content is equal,compareTo() Compare in dictionary order.compareTo() Returns negative, zero or positive numbers, there is no guarantee that only returns -1、0、1。

System.out.println("abc".equals("abc"));
System.out.println("abc".compareTo("abd"));

Find and judge

String text = "abcdefgabc";

System.out.println(text.contains("cde"));
System.out.println(text.startsWith("abc"));
System.out.println(text.endsWith("abc"));
System.out.println(text.indexOf("abc"));
System.out.println(text.lastIndexOf("abc"));

When the specified content cannot be found,indexOf() and lastIndexOf() return -1。

intercept string

substring() The ending index of is not included in the results.

String text = "abcdefg";
System.out.println(text.substring(2));
System.out.println(text.substring(2, 5)); // cde

Replacement, splitting and case conversion

String text = "abcbc";

System.out.println(text.replace("bc", "BC"));
System.out.println(Arrays.toString(text.split("b")));
System.out.println(text.toUpperCase());
System.out.println(text.toLowerCase());
System.out.println("  Java  ".trim());

split() The argument to is a regular expression. When splitting special characters such as dots and vertical lines, you need to escape them according to regular expression rules.

Strings and byte arrays

Character sets should be clearly specified when encoding and decoding.

String text = "你好";
byte[] bytes = text.getBytes(StandardCharsets.UTF_8);
String result = new String(bytes, StandardCharsets.UTF_8);
System.out.println(text.equals(result));

StringBuffer class

StringBuffer Is a sequence of characters whose content is variable. It maintains an expandable buffer internally, and usually does not need to add, insert or delete content such as String Create a large number of intermediate objects repeatedly like splicing.

StringBuffer buffer = new StringBuffer("a");
for (int i = 0; i < 10_000; i++) {
    buffer.append('a');
}
String result = buffer.toString();

Constructors usually reserve extra capacity based on the initial string length. When the buffer space is insufficient, it will be automatically expanded, and the specific expansion strategy belongs to the implementation details.

Common StringBuffer Methods

StringBuffer buffer = new StringBuffer("Java");

buffer.append(" SE");
buffer.insert(4, " 8");
buffer.delete(4, 6);
buffer.reverse();

System.out.println(buffer);

Common methods include append()、insert()、delete()、replace() and reverse()。

StringBuilder class

StringBuilder with StringBuffer Provides similar APIs and also belongs to variable character sequences.

• StringBuffer The main method with synchronization control is suitable for scenarios where multiple threads share the same instance and modify it at the same time.
• StringBuilder Without this synchronization guarantee, the overhead is generally lower in single-threaded scenarios.

Most local string splicing scenarios are preferred StringBuilder。Thread safety cannot be judged solely based on the class name, but also based on whether the object is shared by multiple threads and whether the overall operation is atomic.