修复了一些bug，实现了基本的加减运算数据：使用double类型存储数字，定义一个x表示字符串现在应有的长度，当t改变时使用x调整t的长度，x最高为15
    方法：新建一个字符串，用户按按钮时对其进行改变
    如：t=“0 ”
    用户按1，+，2键
    t=“1+2 ”
    遍历一遍t将数字与符号分别存入两个数组中，之后再进行运算
    监听代码：

    1. public void actionPerformed(ActionEvent e) {
    2.         if(e.getActionCommand().equals("7")) {
    3.             if(x==15) {
    4.                 return;
    5.             }else if(t.charAt(0)=='0') {
    6.                 x=x-1;}
    7.             if(o==1) {
    8.                 x=x-1;
    9.                 o=0;
    10.             }
    11.             StringBuilder strBuilder = new StringBuilder(t);
    12.             strBuilder.setCharAt(x,'7');
    13.             strBuilder.toString();
    14.             t=strBuilder.toString();
    15.             x=x+1;
    16.             s=0;y=0;
    17.             }else if(e.getActionCommand().equals("8")) {
    18.             if(x==15) {
    19.                 return;
    20.             }else if(t.charAt(0)=='0'){
    21.                 x=x-1;}
    22.             if(o==1) {
    23.                 x=x-1;
    24.                 o=0;
    25.             }
    26.             StringBuilder strBuilder = new StringBuilder(t);
    27.             strBuilder.setCharAt(x,'8');
    28.             strBuilder.toString();
    29.             t=strBuilder.toString();
    30.             x=x+1;
    31.             s=0;y=0;
    32.         }else if(e.getActionCommand().equals("9")) {
    33.             if(x==15) {
    34.                 return;
    35.             }else if(t.charAt(0)=='0'){
    36.                 x=x-1;}
    37.             if(o==1) {
    38.                 x=x-1;
    39.                 o=0;
    40.             }
    41.             StringBuilder strBuilder = new StringBuilder(t);
    42.             strBuilder.setCharAt(x,'9');
    43.             strBuilder.toString();
    44.             t=strBuilder.toString();
    45.             x=x+1;
    46.             s=0;y=0;
    47.         }else if(e.getActionCommand().equals("4")) {
    48.             if(x==15) {
    49.                 return;
    50.             }else if(t.charAt(0)=='0'){
    51.                 x=x-1;}
    52.             if(o==1) {
    53.                 x=x-1;
    54.                 o=0;
    55.             }
    56.             StringBuilder strBuilder = new StringBuilder(t);
    57.             strBuilder.setCharAt(x,'4');
    58.             strBuilder.toString();
    59.             t=strBuilder.toString();
    60.             x=x+1;
    61.             s=0;y=0;
    62.         }else if(e.getActionCommand().equals("5")) {
    63.             if(x==15) {
    64.                 return;
    65.             }else if(t.charAt(0)=='0'){
    66.                 x=x-1;}
    67.             if(o==1) {
    68.                 x=x-1;
    69.                 o=0;
    70.             }
    71.             StringBuilder strBuilder = new StringBuilder(t);
    72.             strBuilder.setCharAt(x,'5');
    73.             strBuilder.toString();
    74.             t=strBuilder.toString();
    75.             x=x+1;
    76.             s=0;y=0;
    77.         }else if(e.getActionCommand().equals("6")) {
    78.             if(x==15) {
    79.                 return;
    80.             }else if(t.charAt(0)=='0'){
    81.                 x=x-1;}
    82.             if(o==1) {
    83.                 x=x-1;
    84.                 o=0;
    85.             }
    86.             StringBuilder strBuilder = new StringBuilder(t);
    87.             strBuilder.setCharAt(x,'6');
    88.             strBuilder.toString();
    89.             t=strBuilder.toString();
    90.             x=x+1;
    91.             s=0;y=0;
    92.         }else if(e.getActionCommand().equals("1")) {
    93.             if(x==15) {
    94.                 return;
    95.             }else if(t.charAt(0)=='0'){
    96.                 x=x-1;}
    97.             if(o==1) {
    98.                 x=x-1;
    99.                 o=0;
    100.             }
    101.             StringBuilder strBuilder = new StringBuilder(t);
    102.             strBuilder.setCharAt(x,'1');
    103.             strBuilder.toString();
    104.             t=strBuilder.toString();
    105.             x=x+1;
    106.             s=0;y=0;
    107.         }else if(e.getActionCommand().equals("2")) {
    108.             if(x==15) {
    109.                 return;
    110.             }else if(t.charAt(0)=='0'){
    111.             x=x-1;
    112.             }
    113.             if(o==1) {
    114.                 x=x-1;
    115.                 o=0;
    116.             }
    117.             StringBuilder strBuilder = new StringBuilder(t);
    118.             strBuilder.setCharAt(x,'2');
    119.             strBuilder.toString();
    120.             t=strBuilder.toString();
    121.             x=x+1;
    122.             s=0;y=0;
    123.         }else if(e.getActionCommand().equals("3")) {
    124.             if(x==15) {
    125.                 return;
    126.             }else if(t.charAt(0)=='0'){
    127.                 x=x-1;}
    128.             if(o==1) {
    129.                 x=x-1;
    130.                 o=0;
    131.             }
    132.             StringBuilder strBuilder = new StringBuilder(t);
    133.             strBuilder.setCharAt(x,'3');
    134.             strBuilder.toString();
    135.             t=strBuilder.toString();
    136.             x=x+1;
    137.             s=0;y=0;
    138.         }else if(e.getActionCommand().equals("0")) {
    139.             if(x==15) {
    140.                 return;
    141.             }else if(t.charAt(0)=='0'){
    142.                 return;
    143.                 }else {
    144.                     if(t.charAt(x-1)=='+') {
    145.                         o=1;
    146.                     }
    147.             StringBuilder strBuilder = new StringBuilder(t);
    148.             strBuilder.setCharAt(x,'0');
    149.             strBuilder.toString();
    150.             t=strBuilder.toString();
    151.             x=x+1;
    152.             s=0;y=0;
    153.             }
    154.         }else if(e.getActionCommand().equals("+")) {
    155.             if(x==15||x==14) {
    156.                 return;
    157.             }else if(t.charAt(0)=='0'||s==1) {
    158.                 return;
    159.             }else {
    160.                 if(y==1){
    161.                     x=x-1;
    162.                 }
    163.                 StringBuilder strBuilder = new StringBuilder(t);
    164.                 strBuilder.setCharAt(x,'+');
    165.                 strBuilder.toString();
    166.                 t=strBuilder.toString();
    167.                 s=1;
    168.                 y=0;
    169.                 x=x+1;
    170.             }
    171.         }else if(e.getActionCommand().equals("-")) {
    172.             if(x==15||x==14) {
    173.                 return;
    174.             }else if(t.charAt(0)=='0'||y==1) {
    175.                 return;
    176.             }else
    177.             {
    178.                 if(s==1){
    179.                     x=x-1;
    180.                 }
    181.                 StringBuilder strBuilder = new StringBuilder(t);
    182.                 strBuilder.setCharAt(x,'-');
    183.                 strBuilder.toString();
    184.                 t=strBuilder.toString();
    185.                 y=1;
    186.                 s=0;
    187.                 x=x+1;
    188.             }
    189.         }else if(e.getActionCommand().equals("=")) {
    190.             int j=0;
    191.             double []a=new double[10];
    192.             int b=0;
    193.             String[] t2=new String[10];
    194.             while(t.charAt(j)!=' ') {
    195.                 String t1="0                    ";
    196.             for(int i=j;i<16;i++) {
    197.             if(t.charAt(i)>='0'&&t.charAt(i)<='9') {
    198.                 StringBuilder strBuilder1 = new StringBuilder(t1);
    199.                 strBuilder1.setCharAt(i-j,t.charAt(i));
    200.                 strBuilder1.toString();
    201.                 t1=strBuilder1.toString();
    202.             }else {
    203.                 c[b]=t.charAt(i);
    204.                 j=i+1;
    205.                 break;
    206.                 }
    207.             }
    208.             double all=0;
    209.             for(int g=0;g<t1.replace(" ","").length();g++) {
    210.                double u=(int)Math.pow(10,t1.replace(" ","").length()-g-1);
    211.                 all=all+u*Integer.parseInt(t1.charAt(g)+"");
    212.             }
    213.             t2[b]=t1;
    214.             a[b]=all;
    215.             b=b+1;
    216.             }
    217.             if(sum==0) {
    218.                 sum=a[0];
    219.             }
    220.             for(int l=0;l<b;l++) {
    221.             if(c[l]=='+') {
    222.             sum=sum+a[l+1];
    223.             }else if(c[l]=='-') {
    224.                 sum=sum-a[l+1];
    225.             }
    226.             }
    227.             int f=0;
    228.             for(int m=0;m<b;m++) {
    229.             if(t2[m].replace(" ","").length()>10) {
    230.                 t="error";
    231.                 sum=0;
    232.                 f=1;
    233.             }
    234.             }
    235.             if(f==0) {
    236.             t=String.valueOf(df.format(sum));
    237.             }
    238.             x=t.replace(" ", "").length();
    239.             for(int h=0;h<16-(x-1);h++) {
    240.             t=t+" ";
    241.             }
    242.             s=0;y=0;
    243.         }else if(e.getActionCommand().equals("Replay")) {
    244.             t="0                     ";
    245.             x=1;
    246.             sum=0;
    247.         }
    249.             }