深入學習java原始碼之StringBuilder.indexOf()與StringBuilder.reverse()

java的toString()方法

在Java中每個類都預設繼承Object類，除非宣告繼承某個類。而Object類中有一個叫做toString的方法。該方法返回的是該Java物件的記憶體地址經過雜湊演算法得出的int型別的值在轉換成十六進位制。這個輸出的結果可以等同的看作Java物件在堆中的記憶體地址。

我們知道，在輸出一個類的時候，如果沒有重寫父類的toString方法的話，列印的將會是[email protected],即

public String toString() {
        return getClass().getName() + "@" + Integer.toHexString(hashCode());
}
 

所以只有子類重寫父類的toString()方法之後，才會列印你想輸出的資訊。

System.out.println的原始碼：

public void println(Object x) {
        String s = String.valueOf(x);
        synchronized (this) {
            print(s);
            newLine();
        }
    }

String.valueOf(),

public static String valueOf(Object obj) {
        return (obj == null) ? "null" : obj.toString();
}
 

java原始碼

package java.lang;

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{

    static final long serialVersionUID = 4383685877147921099L;

    public StringBuilder() {
        super(16);
    }

    public StringBuilder(int capacity) {
        super(capacity);
    }

    public StringBuilder(String str) {
        super(str.length() + 16);
        append(str);
    }

    public StringBuilder(CharSequence seq) {
        this(seq.length() + 16);
        append(seq);
    }
	
    @Override
    public int indexOf(String str) {
        return super.indexOf(str);
    }

    @Override
    public int indexOf(String str, int fromIndex) {
        return super.indexOf(str, fromIndex);
    }

    @Override
    public int lastIndexOf(String str) {
        return super.lastIndexOf(str);
    }

    @Override
    public int lastIndexOf(String str, int fromIndex) {
        return super.lastIndexOf(str, fromIndex);
    }

    @Override
    public StringBuilder reverse() {
        super.reverse();
        return this;
    }

    @Override
    public String toString() {
        // Create a copy, don't share the array
        return new String(value, 0, count);
    }
	
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        s.defaultWriteObject();
        s.writeInt(count);
        s.writeObject(value);
    }
	
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();
        count = s.readInt();
        value = (char[]) s.readObject();
    }	
}

package java.lang;

import sun.misc.FloatingDecimal;
import java.util.Arrays;

abstract class AbstractStringBuilder implements Appendable, CharSequence {

    char[] value;
	
    int count;	
	
    AbstractStringBuilder() {
    }

    AbstractStringBuilder(int capacity) {
        value = new char[capacity];
    }

    @Override
    public int length() {
        return count;
    }

    public int capacity() {
        return value.length;
    }

    private int newCapacity(int minCapacity) {
        // overflow-conscious code
        int newCapacity = (value.length << 1) + 2;
        if (newCapacity - minCapacity < 0) {
            newCapacity = minCapacity;
        }
        return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
            ? hugeCapacity(minCapacity)
            : newCapacity;
    }	
	
    private void ensureCapacityInternal(int minimumCapacity) {
        // overflow-conscious code
        if (minimumCapacity - value.length > 0) {
            value = Arrays.copyOf(value,
                    newCapacity(minimumCapacity));
        }
    }	

    public void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)
    {
        if (srcBegin < 0)
            throw new StringIndexOutOfBoundsException(srcBegin);
        if ((srcEnd < 0) || (srcEnd > count))
            throw new StringIndexOutOfBoundsException(srcEnd);
        if (srcBegin > srcEnd)
            throw new StringIndexOutOfBoundsException("srcBegin > srcEnd");
        System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
    }
	
    public int indexOf(String str) {
        return indexOf(str, 0);
    }
	
    public int indexOf(String str, int fromIndex) {
        return String.indexOf(value, 0, count, str, fromIndex);
    }
	
    public int lastIndexOf(String str) {
        return lastIndexOf(str, count);
    }

    public int lastIndexOf(String str, int fromIndex) {
        return String.lastIndexOf(value, 0, count, str, fromIndex);
    }

    public AbstractStringBuilder reverse() {
        boolean hasSurrogates = false;
        int n = count - 1;
        for (int j = (n-1) >> 1; j >= 0; j--) {
            int k = n - j;
            char cj = value[j];
            char ck = value[k];
            value[j] = ck;
            value[k] = cj;
            if (Character.isSurrogate(cj) ||
                Character.isSurrogate(ck)) {
                hasSurrogates = true;
            }
        }
        if (hasSurrogates) {
            reverseAllValidSurrogatePairs();
        }
        return this;
    }	
	
    private void reverseAllValidSurrogatePairs() {
        for (int i = 0; i < count - 1; i++) {
            char c2 = value[i];
            if (Character.isLowSurrogate(c2)) {
                char c1 = value[i + 1];
                if (Character.isHighSurrogate(c1)) {
                    value[i++] = c1;
                    value[i] = c2;
                }
            }
        }
    }
	
    public String substring(int start) {
        return substring(start, count);
    }
	
    @Override
    public CharSequence subSequence(int start, int end) {
        return substring(start, end);
    }

    public String substring(int start, int end) {
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (end > count)
            throw new StringIndexOutOfBoundsException(end);
        if (start > end)
            throw new StringIndexOutOfBoundsException(end - start);
        return new String(value, start, end - start);
    }	
	
    @Override
    public abstract String toString();
	
    final char[] getValue() {
        return value;
    }
	
}