private static void test(){
        String test = "123\\456";
        String str = test.substring(0, test.indexOf("\\"));
        System.out.println(str);
    }

------解决方案--------------------
String tes = "asdd//sdfsd//sdf";
String res = (tes.split("//"))[0];

------解决方案--------------------
import java.util.regex.*;

public class Hello {
	public static void main(String[] args) {
		String str = "I am a boy, \\and I love beauty!\\haha";
//运用正则表达式
		Pattern p = Pattern.compile("\\\\.*");
		Matcher m = p.matcher(str);
		System.out.println(m.replaceAll(""));
	}
}

------解决方案--------------------
import java.util.regex.*;

public class Hello {
	public static void main(String[] args) {
		String str = "I am a boy, \\and I love beauty!\\haha";
		System.out.println(str.replaceAll("\\\\.*", "")); //正则表达式
	}
}

------解决方案--------------------
HTML code

        String str="asdd//sdfsd//sdf "; 
        str=str.replaceAll("//.*","");
        System.out.println(str);