When using the StringBuffer class, the StringBuffer object itself is manipulated each time, rather than generating a new object, so it is recommended to use StringBuffer if you need to modify the string. A StringBuffer is described as follows:

Char [] array (); char[] array ()

Example 1: Large string concatenation

In the process of program development, we often encounter the situation of String concatenation, convenient and direct way is through the + symbol to achieve the purpose, but this way to achieve the efficiency is relatively low, and each execution will create a String object, that is time consuming, and waste space. Using the StringBuilder class avoids this problem

String s0 = "abcdefghigklmnopqrstuvwxyz";
StringBuilder sb = new StringBuilder();
long t = System.currentTimeMillis();
for (int i = 0; i < 100000; i++) {
	sb.append(s0);
}
System.out.println(System.currentTimeMillis() - t);
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Running results:

You can see that the letters A-Z were concatenated 100,000 times in 22 milliseconds



Create an object

StringBuilder sb = new StringBuilder();
或者
StringBuilder sb = new StringBuilder("abc");
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Commonly used method

Append () is used to append characters to the end of a character sequence, efficient string concatenation

Delete (int start,int end) Deletes characters from a string from start to end, that is, [start,end)

DeleteCharAt (int index) Deletes a single character

Insert (int index, substring) Function: Insert a substring at index position

Replace (int start,int end,String STR) Replaces [start,end) with the specified String STR

Reverse () does: reverse

SetCharAt (int index,char ch) replaces the index position with the character CH

ToString () converts toString

Example 2: StringBuilder method exercise

		StringBuilder sb = new StringBuilder("abc");
        sb.append("def")//abcdef
                .append("ghi")//abcdefghi
                .insert(2."jkl")//abjklcdefghi
                .replace(4.6."mnop")//abjkmnopdefghi
                .reverse()//ihgfedponmkjba
                .delete(2.5)//ihdponmkjba
                .deleteCharAt(0)//hdponmkjba
                .setCharAt(2.The '*');//hd*onmkjba
        System.out.println(sb);
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Running output:

hd*onmkjba

Example 3: Check your hobbies

The following effects need to be achieved:

xml

<TableLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:stretchColumns="0">

    <TableRow>

        <CheckBox
            android:onClick="doClick"
            android:text="Football" />

        <CheckBox
            android:onClick="doClick"
            android:text="Basketball" />

        <CheckBox
            android:onClick="doClick"
            android:text="Balloon" />
    </TableRow>

    <TableRow>

        <CheckBox
            android:onClick="doClick"
            android:text=Pop music />

        <CheckBox
            android:onClick="doClick"
            android:text="Classical music" />

        <CheckBox
            android:onClick="doClick"
            android:text="Little Moon and moon" />
    </TableRow>

    <TableRow>

        <CheckBox
            android:onClick="doClick"
            android:text="Beijing Opera" />

        <CheckBox
            android:onClick="doClick"
            android:text="Drama" />

        <CheckBox
            android:onClick="doClick"
            android:text="Cup with" />
    </TableRow>

    <TableRow>

        <CheckBox
            android:onClick="doClick"
            android:text="Smoking" />

        <CheckBox
            android:onClick="doClick"
            android:text="Drinking" />

        <CheckBox
            android:onClick="doClick"
            android:text="Permed hair" />
    </TableRow>

    <TextView
        android:id="@+id/textView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textColor="# 222"
        android:textSize="18sp"
        android:gravity="center"
        android:layout_marginTop="20dp"/>
</TableLayout>
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The Activity of

public class MainActivity extends AppCompatActivity {
    private TextView textView;
    private StringBuilder sb = new StringBuilder();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textView = findViewById(R.id.textView);
    }

    / * * the View all interface controls the parent type * * | | - EditText - TextView * | -... * * /
    public void doClick(View view) {
        CheckBox checkBox = (CheckBox) view;
        String s = checkBox.getText().toString();
        if (checkBox.isChecked()) {/ / check
            if(sb.length() ! =0) {
                // Add a link to the existing content.
                sb.append(",");
            }
            sb.append(s);
        } else {/ / cancel
            // Middle case: CCC
            //aaa,bbb,ccc,ddd,eee
            //start:8 end:12

            // At the end: CCC
            //aaa,bbb,ccc
            //start:8 end:12
            //end > 11

            // start with CCC
            //ccc,aaa,bbb
            //start:0 end:4

            // Unique: CCC
            //ccc
            //start:0 end:4
            int start = sb.indexOf(s);/ / 8
            int end = start + s.length() + 1;//(delete commas, delete is left closed right open range, so add 1)
            //end > sb.length
            // Delete the last CCC, there is no comma after it, so end--
            // Delete CCC as well as the preceding comma, so start--
            //
            if (end > sb.length()) {
                if(start ! =0) { start--; } end--; } sb.delete(start, end); } textView.setText(sb); }}Copy the code