(基本数据类型+包装类) 和 String 相互转化

1. 基本数据类型,包装类 -> String

1.1. 之前的做法: 连接运算

package com.beyondx.java;
import org.junit.Test;

public class WrapperTest {
    // 基本数据类型, 包装类 -> String
    @Test
    public void test5() {
        int num1 = 10;
        String str1 = num1 + "";
    }
}

1.2. 现在的做法

基本数据类型, 包装类 -> String, 调用 String重载的 valueOf(Xxx xxx)

package com.beyondx.java;
import org.junit.Test;

public class WrapperTest {
    @Test
    public void test6() {
        // 1.基本数据类型 -> String
        float f = 12.3f;
        String str1 = String.valueOf(f);
        System.out.println(str1);
        
        // 2.包装类 -> String
        Double d = new Double(12.4);
        String str2 = String.valueOf(d);
        System.out.println(str2);
    }
}

java.lang.String部分源码

public final class String implements java.io.Serializable, Comparable<String>, CharSequence {
    // 很多构造器
    public String() {
    }
    public String(String original) {}
    public String(char value[]) {}
    public String(char value[], int offset, int count) {}
    public String(int[] codePoints, int offset, int count) {}
    public String(byte ascii[], int hibyte, int offset, int count) {}
    public String(byte ascii[], int hibyte) {}
    public String(byte bytes[], int offset, int length, String charsetName) {}
    public String(byte bytes[], int offset, int length, Charset charset) {}
    public String(byte bytes[], String charsetName) {}
    public String(byte bytes[], Charset charset) {}
    public String(byte bytes[], int offset, int length) {}
    public String(byte bytes[]) {}
    public String(StringBuffer buffer) {}
    public String(StringBuilder builder) {}
    
    public static String valueOf(Object obj) {}
    public static String valueOf(char data[]) {}
    public static String valueOf(char data[], int offset, int count) {}
    public static String valueOf(boolean b) {}
    public static String valueOf(char c) {}
    public static String valueOf(int i) {}
    public static String valueOf(long l) {}
    public static String valueOf(float f) {}
    public static String valueOf(double d) {}
}

2. String -> 基本数据类型, 包装类

String -> 基本数据类型, 包装类, 调用包装类的 parseXxx(String s)

package com.beyondx.java;
import org.junit.test;

public class WrapperTest {
    @Test
    public void test6() {
        String str1 = "123";
        int num = Integer.parseInt(str1);
        System.out.println(num + 1);
        
        String str2 = "true";
        boolean b1 = Boolean.parseBoolean(str2);
        System.out.println(b1);
    }
}

java.lang.Integer部分源码

public final class Integer extends Number implements Comparable<Integer> {
    // 一些方法
    public static int parseInt(String s, int radix) throws NumberFormatException {}
    public static int parseInt(String s) throws NumberFormatException {}
    public static int parseUnsignedInt(String s, int radix) throws NumberFormatException {}
    public static int parseUnsignedInt(String s) throws NumberFormatException {}
}